Is it possible to only enable signatures for particular categories?
I’m afraid that’s not currently possible, but I’ve moved your suggestion into the Contribute > UX category to see if other people have also felt a need for this. 
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It would seem like you could css hide them by category?
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That’s true. I should’ve said, ‘in the UI’.
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Okay, thanks. I am trying to stumble my way through learning CSS so we’ll see if I can sort out making that change!
There are some topics and/or theme components that do stuff in certain categories, so you might search for those.
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