1. Don't talk to them if possible. Muting everyone is ok, might not result in the greatest synergy, but it's better than tilting from trash talk.
2. If they add you post game to talk trash, don't accept or decline the request, just let it sit there for a week or so until they forget about you.
3. If you must talk to them, avoid antagonizing them, avoid criticizing them unless they SPECIFICALLY ask for criticism.
4. Don't be toxic yourself. You attract the kind of person that you are.
If I've missed anything, feel free to comment.