Ah, the delicate art of telling someone they're not funny without crushing their spirit. Well, you can always try using subtle hints like gently chuckling at their jokes and saying things like, "Oh, that's an interesting one" or "You certainly have a unique sense of humor." Alternatively, you can divert the topic by sharing a really hilarious joke yourself and hope they get the hint. Just remember to be kind and tactful while reminding them that comedy is not their strongest suit. Good luck!