You have a few options in such a case.
#1) You must still respond to them, and you can tell them that you will return their vote once they have provided you with the number or other proof of vote.
#2) You can hit the report button on their post and report them for not following rules if a CG staff member has not already warned them, BUT you MUST still respond to them that you will RTF when they have provided their proof of vote.
#3) If you want to you can just return the vote. We would ask that if a CG staff member has not already warned them that you still hit the report button on their post and report them for not following rules.
The ONLY time you are not required to return the vote or ask for proof then RTF is in the case of a Restricted User (orange names) asking for a return favor. In this case please IMMEDIATELY hit the report button to alert the Contest Galaxy Staff, if their post has not already been censored by a mod or admin.
Please remember, two wrongs do NOT make a right.