How do you tactfully handle friends and/or relatives who verbally discipline your children in your presence?
The short answer: Be assertive.
Seriously. There's nothing inherently tactless about expressing your feelings to someone, or clearly defining your boundaries with them. In fact, it's almost tactless not to, because you're giving them inaccurate information concerning how to respect you and your children. Just be considerate and polite while you're doing it, and you should be fine.
One of the Big Ideas with assertiveness is to use I Statements. "I would like you to respect me and my parenting goals by not parenting my children." "I don't feel comfortable with you giving parenting messages to my children, and I would like you to respect that." Clear, calm, polite and direct statements concerning your feelings and boundaries are perfectly appropriate to express, and no sane person would suggest that you're "not allowed" to tell someone how you feel about their behavior. Make sure that you phrase it in a way that isn't likely to be misinterpreted as an accusation, or as anger (for example, "I would appreciate it if you wouldn't try to raise my kids." would probably be misinterpreted that way, just because of societally-accepted implications of "I would appreciate it if..") If they come back at you with an excuse why their behavior is okay or warranted, give them another I Statement. "I don't think that's your choice to make.", or "I don't see why you believe it is appropriate to attempt to parent my children, and I don't feel comfortable with that. I would like you to respect me and my children, and allow me to parent my children myself." You could even start out with, "I understand that you would like to make sure my children grow up well, but..." and go on from there. Or you could just use the Broken Record approach, repeating your original message to them (which they've just deflected with an excuse) until they take heed. These are basic assertiveness approaches, and there are more in-depth tutorials for assertiveness available online. Good luck! =)