Well, if she's making a mess with whatever's in her diaper, I'd put her in a time out in the tub (easy to clean, harder to get out) immediately after she does it each time. Don't give her a big frustrated reaction. Just be really cool and say something like, "Uh oh, looks like a little tub time." And then take her directly to the bathtub and leave her in the tub for a couple minutes as a time out. She'll figure out quickly that it's not so fun to do that. And then be really really sweet and fun when you take her out, so she knows how much fun it is to be out of time out and how boring it is when she's in time out.