Cindy's "gentle discouragement" is a possible but I think I would be a little more direct--"I'm sure you didn't understand sign-up protocol; you have to be invited to join a team and since we have a full team per our plans, I am afraid you will have to switch teams. Why don't you check with the
League coordinator and she/he will help you with that. You won't lose your fee."
I've been a team captain for around twenty years and never had that happen. I have had a couple of equally tough ones where a player contacted me and basically begged to be on the team!