1) Don’t be Unbearable
This is almost certainly the hypothetical most-effective strategy, attacking the problem upstream so that it never actually becomes a problem. The basic premise is that people are generally only triggered into called someone a ‘bandwagoner’ if they’re acting obnoxious, over the top or in your face, especially when celebrating their team’s successes or insulting other teams’ failures.
If you can cut out that part of your persona, then you should ideally be in the clear. However, that is essentially the entire fun of being a sports fan. I’ve never once met a single one that doesn’t act exactly like that so this likely isn’t an option for the overwhelming majority of you.