On several StackExchange sites, I still have less than 15 reputation, which means I cannot vote on these sites. But the pages still prompt me to give an upvote if I find the answer useful. It would be really great if you could not only remove the bogus invitation to vote, but also remove the voting buttons entirely. If I'm not welcome to vote, don't invite me to vote.
Removing the voting buttons is a bad idea. There would be many users who ask why they are not seeing the voting buttons in some sites, but they do see them in other sites.
I would rather show them disabled, with a tooltip that explains why they are disabled. This is what happens on the meta site, when a user cannot edit a post because her/his reputation is still low to allow that.