I'm a sig-nazi too.
On ProjectGuitar.com, we allow (2) lines of text. No images. If you're a site-contribuiting vendor, you may include a text link to your site. Large font sizes and other things are trimmed as they are discovered.
Members get 1 warning. 2nd warning is a 2-week suspension. (I'm not called 'The Moderator From Hell' over there because I'm all puppies and cupcakes.)
People bitch and moan at first, but soon realize how nice it is to not scroll thru 8 pages of signatures, banner-atures, and spam-atures to get to the one post that says:
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+1 to Darren's post.