That sounds like a messy situation. If this coach is causing trouble, you shouldn't allow him/her to coach on your club's ice. The board could probably get around this by saying that skaters must be members for the full year to get the medal. From your first post, it sounds like they passed their tests while still members of the old club - in that case, just say that they weren't members when they passed the test and that's that.
Great suggestions, all. Can you give a coach notice that s/he isn't going to be allowed to coach on club ice?
S/he might end up on the rink freestyles, which you can't do anything about, but our club had the option of declining coaches.
Can you review the bylaws and/or prior meeting minutes to see if there's a precedent for the Senior skater? Or, if it's something that might happen next year, propose an amendment to the bylaws saying that pro-rated memberships aren't eligible for the award or they have to pay for it.
If it's less than $50, just write it off to good will and use it as a PR opportunity.