First, before people start yelling "solar storms!" just bear in mind that even the craziest geomagnetic phenomena might not be effective against lower frequencies traveling long distances.
How this question is best answered depends on the forms of communication we're talking about. If you limit your technology to specific forms of communication (i.e., what frequencies we're dealing with), the constraints are easier to define, and it's easier to give you more specifics. (Microwave frequencies behave differently than FM, etc.)
As your question implies, you are already aware that some forms of radio waves will reach distances beyond the line-of-sight of the transmitter because of how they "bounce" off the ionosphere. But they can also travel long distances as ground waves. This Wikipedia article has some good info. So, it may be best to think of this question in two parts:
No transmissions should make it to orbit...
Generally speaking, in theory, I suppose if you had a ridiculously ionized layer of atmosphere, it could conceivably block most if not all signals from reaching orbit, so that solves half of your problem. (The science is a bit less simple than I'm making it sound, but that's the basic idea.) But again, the frequencies involved will affect this.
Additionally, the composition of the atmosphere itself can affect signal propagation. Altering the content of the various atmospheric gases could give you the effects you're looking for; different gases react differently to EM. Of course, too much alteration and the air will become unbreathable, or air pressure will become too high, and people will need EV suits. I recommend visiting the Physics or Space Exploration Stack Exchange sites for more info in these regards.
...or over the horizon line.
I would consider using terrain as a way to limit your range. Bodies of water and rugged, mountainous landscapes absorb a lot of EM radiation. Volcanic activity producing high amounts of ash can help, but anything short of a cataclysmic eruption would probably not cause much interference for long, and certainly not over a wide area. So, probably too unreliable for you to count on.
Note also that some weather can actually work against you and improve the signal range.