I'm guessing you're on PC like I am. Hackers have been a thing unfortunately and while EA recently brought up reporting hackers again, their track record hasn't been great. I still recommend reporting them, especially if you can get a screenshot of them playing bosses or other illegal characters.
This is far too common a problem, especially for a 5 year old game. But as I've said before, we can still buy the game, play the game, and buy coins/upgrades for it, so a minimum expectation of dealing with hackers and other problem players is to be met, and it's currently not doing so.
