By taking Mass Effect as a reference you can also have the technological level that triggers the annihilation.
In the game, as the Reapers destroy humans and other sentient species, they also avoid the Yahg and let them survive for the next cycle. The Yahg technological level is described as:
When Parnack was discovered by Citadel Council surveyors in 2125 CE,
the yahg were a pre-spaceflight civilization with technology
equivalent to that of 20th century Earth.
So two elements seem important here:
- pre-space flight
- 20th human century-like
Long range: As @Twelfth and @JDługosz explained you need to protect yourself against long range detection. However 20th human century-like doesn't imply no emission at all but a lower energy level and bad quality and speed.
In addition to other answers about reducing the EM radiation I suggest to send fake signals. Indeed, as the Reapers might decode the video and audio signals from earth, you can send very bad quality images and audio picturing a way less developed world.
You should also stop all probes sent in outer space from emitting and if possible destroy them.
Fly-by: If the Reapers decide to check by them-self and to go closer to Earth the situation becomes a bit more complicated. You need to get rid of all spaceflight technology proofs, including satellite, scraps, moon landers, rovers and probes on other planets ... (And definitely get rid of the EM shield if you built it)
As they can take pictures of the ground you need to cover and disguise all technological advanced buildings and sites. Using the same kind of trick of Ghost armies could be very useful (without forgetting taking in account the scanning technology level of the Reapers).
Agents: If the Reapers send agents on the ground to check ... I can't see other solution than actually downgrading your technology. Difficult to figure out how to remove everything on Earth related to modern technology, including waste (if we could we would already have solved a big part of our ecological problems).
If you can achieve that, time capsule with blueprints and instructions to rebuild our technology could be handy once the Reaper have left.