You should never be forced to play a game that you don't want to play. If your priority at warhammer is to win, then you are going to have a bad time vs toxic players. If your priority at warhammer is to have a fun game, then you are in control.
Pickup games - Easily avoided. Just say no if he asks. Better yet, plan your games ahead of time with other people you specifically enjoy playing. (always the best method IMHO). Strangely, many toxic players I know tend to avoid scheduling games in advance and simply like to lurk about the venue asking people for games.
Small-Med tournaments - Always grudge someone round one. Almost all tournaments allow this as first round pairings are not terribly important. This will cut your odds of playing him by a third. If he is a WAAC netlister and you are a casual fluff player, then hopefully he will shoot off to the top tables and be far away from you.
Med-Large tournaments - Just quietly inform the TO beforehand that you and the player do not get along, and that if paired, you will politely concede the game or will conveniently leave the venue for the round. Most TOs will prefer the non-confrontational approach and will make sure to simply pair you with someone else. Larger tournaments usually have more wiggle room in pairings. Furthermore, if your friends share the same opinion, have them speak up as well.