For contests sometimes you're supposed to have disclaimers like "void where prohibited" but I was wondering if there's anything specific you need to know for giving away crypto.
Generally, just say that you're reserving all rights to change the terms whenever you want when you are conducting a giveaway.
There shouldn't be much legal repercussion once you state this, especially considering that it's a giveaway and not any time of formal work contract. And as long as your actions are justified, in terms of for example, not paying a certain user in the giveaway because he cheated or whatever, then you most likely won't face any issues on this forum either.
Only thing I could think of is if you're giving away your ICO tokens, in which case you may want to check whether or not you need to legally do KYC before you start.