Yup, you could disable signatures/avatars in the forum settings, but they're not considered ads and won't be disabled by ticking "Disable ads" in the profile settings. Similarly, I don't think any ad-blockers would view signatures as ads (unless specifically designed/modified for this purpose). I've been using Brave browser for a few years now, and it wouldn't block signatures even when you pick the strictest option: "Aggressively block trackers & ads".
That's a really nice advantage of signatures over classic ads which is rarely mentioned. I think campaign managers should use this argument when pitching to potential clients.