Alright. I've decided to transform my set of comments into an answer. Well.. the logic in @kingledion answer is pretty nice. From there, if a pirate ship is undetectable, then it might steal a ship with no one noticing. Also, there are plenty of nice answers here not relying on pirates having a ship and intercepting. Mine will do.
Definition 1: We will define anything with the potential of government quick response as GQRs. They might be stations, bases, planets, moons, patrol ships, whatever.
Doing piracy: There are two places to attack: (1) Near GQRs or (2) not near GQRs. =). (1) In near GQRs, goverments will certainly quickly respond to any distress call (by definition), with high speed fighters or worse. If (2), goverment will not quickly respond (by definition!).
Proposition 2: It is risky to do piracy near GQRs. Proof: We do attack near GQRs. High speed fighters will quickly intercept us. If we are detectable, we are doomed. If we are undetectable, they will investigate, and maybe see us with naked us. Or fire blindly. Thus its risky. We conclude the proof.
The solution is simple: Be non-near GQRs. If you are detectable, they will see you coming, and will prepare for you. (Either weapons in the cargoship, or deviate from you if you get too closed). Dangerous, risky, and has many possibilities of going wrong. We hereby need to be undetectable.
The Ship:
Have a pirate ship P. Now, cover full whole with nice 100% re-directing meta materials. Incident radiation will be redirected and making it invisible to radar. Thermal signature can be extinguished by a rather simple method.
The Invisibility:
Proposition 3: Pirate meta-material hull cannot receive heat.
Proof: There is only two ways to receive heat. (1) From outside, (2) from inside.
(1) By hypothesis, incoming radiation is redirected. Since there is no convection/conductance in space, we are done.
(2) Separate inside with nice good insulators. It will insulate the heat for the amount of time needed: few months. Then we are done. This concludes the proof.
Proposition 4: If we wait long enough, the pirate ship hull will not emit detectable thermal radiation.
Proof: We know hull will not receive heat. But it will emit heat by blackbody radiation, at expense of its own temperature. Which will decrease, and emit less and less blackbody radiation. Solution is then simple: A pirate will launch its ship, and wait a little. =). By my calculations, given a ship with outside hull area $A$, hull mass $m$, hull specific heat $c$, and initial temperature $T_0$, the time needed to drop temperature from $T_0$ to $T_0/2$ is:
$$
t_c = \frac{7mc}{3T_0^3 A\sigma}
$$
Where $\sigma$ is Stefan-Boltzmann constant. If we plug some OK values, we will find out no much is needed to wait (a few days only). The invisibility thus works. Now, we need to make sure you survive the trip.
The Heat Problem: The ship cannot and will not have heat exchange with outside. You will then risk increasing inside temperature. This can be done creating an inside atmosphere of plenty heat capacity. If you produce power $P$, then temperature inside will increase:
$$
\frac{dT}{dt} = \frac{P}{mc}
$$
Method 01: Then, if we have a 10x10x10 place for you, filled with water, and if we produce 1000W, it will increase interior temperature at a rate of $1.6$ kelvins per month. (Yes, you wear some under-water cloth to ensure your survival). (It is evident that you won't fill with water and launch. You will launch and fill with water). But this is a bad method.
Method 02: In a nicer method, we manage heat. We create to compartments, one for you and one for heat. And we transfer all waste heat from your room, to heat room. The rooms are separated by insulators. The transfer can be some refrigerator, or peltier device. In heat room, we place something to store the heat (say.. water!). Or whatever high heat capacity material one prefers. The previous calculations will then apply to the heat room.
Method 03: A much more better way (suggested by @Samuel in the comments), is to have blackbody radiation out (perhaps even from the heat room). But, before let it out, transform it in a unidirectional beam (with set of mirrors or lenses or whatever), and point it to a direction perpendicular to the orbital plane. No one is going to see it. You can further assume no one is cloaked, and then you know everyone's position. And then you can point it so no one will detect. =).
Equipped with an undetectable ship, you can match speed of some other ship. This can be done by selecting target while in near GQRs, set up nice burst (no chemical out of preference...) and patiently wait. Burst could be, say.. some reaction to produce $H_2O_2$ and expel it. It will produce barely detectable infrared signature.
Pirating: Method 01: Once you are very near the ship. (1) Disable broadcast capabilities. Maybe shoting all antenas at the same time, or almost at the same time. (2) Open a hole in incoming ship (maybe spotting some glass and breaking it) (out of preference, the main bridge glass), invade it, take control. (3) Now you can program the ship to continue the trip normally. When arrive in planet you set it to go, have it programmed to land in middle of no where, where some of your colleagues will be waiting in there. Steal, and go away. Simple. =).
Pirating: Method 02: Contact incoming ship and say you will blow it to pieces unless some money is transferred to some account. If money is not transferred, well, blow it to pieces. =) (maybe some torpedo).
Conclusion: There are plenty more and more methods to make it possible. The key is to adapt. Applying these strategies again and again, the government undoubtedly will engage successful countermeasures. No strategy survive forever. Adapt. In sea, maybe the equivalent of this, would be pirates in submarines. =D. Well.. If someone have spot some mistake let me know. =).
