Just using the word "toxic" as a binary classifier of "things I don't like" is a problem. That includes people not being positive enough.
I think few people are that bad that you need to isolate yourself from them. In most cases, you can learn not to take their criticism, not to give into their games...
e.g. You've had a bad day at work:
Negative: Being a bit withdrawn and avoiding conversation
Toxically negative: Shouting at your wife and kids
Maybe there could also be a side definition for running competitively in a nuclear contamination zone though!