Can we get an option added to just not show gif avatars? I'll try your ad-blocker solution.
I'm not aware of an option within vB to only show non-gif avatars on a per-user basis.
IMO, either the ad-blocker solution I suggested or the GIF blocker extension (not sure if this allows an explicit blacklist or not, you'd have to read up on the extension's documentation) that iParanormal suggested are going to be your best/easiest options.