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When I choose some forums and hit update I assume that this enables those forums for karma voting, but when the list reloads it doesn't give you any indication of which are active and which aren't.
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Hmmm it should select them if karma is on. I'll go and check.

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Also, if there is a way to turn off karma voting for a forum?
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You answered that yourself above :) ...the forums you select on the Karma Forums page will allow voting, the others won't.