There are prisons all over the world and they are used to keep criminals and murderers away from the rest of society. The problem is that there are criminals that are not human and they are more of a threat than any of the most vicious and evil human criminals in the world. These alien outlaws are all in a very special secret prison that has maximum security meant to hold the most dangerous of species.
The only real problem with deporting these species back to their home planets is that they could use the information they already have to communicate to other aliens about any strategies that they want to use to come back to Earth and take over the planet. This is the reason why we keep them away from any kind of communication and the reason they are kept as prisoners on Earth indefinitely.