I've gotten feedback for a complete draft of my WIP fantasy novel. Overall it's looking good, but a couple of friends noticed a significant loophole in the system of magic I use in the book. The book introduces the basic rules of how magic works in my fantasy setting. Without making this too specific about my particular magic ruleset, reviewers liked the magic system, but these two figured out the same basic loophole, which basically makes it possible to gain a ridiculous amount of magical power (way beyond anything anybody has in my book, and certainly way beyond what my protagonists have).
I might be able to find a small tweak to the rules that could close the loophole. But if I don't find something simple, easy to explain, that makes sense with the rest - is there anything I can do to solve a major flaw in my magic system, without needing to rewrite the whole system and novel from the start?