This is going to sound mean... but when my 12 month old stand up, I dump a cup full of water on her head. She HATES that. I always cover her face with my hand when I am dumping water to rinse her, and I dump it slowly, so she isn't scared of it or anything like that... she just doesn't like the sudden rush of water over her face. When she started standing in the tub at 9 months, I would initially just tell her very firmly "NO! We need to sit!" and make her sit back down. It worked for a while, but eventually she really started fighting that. Then I started taking her out of the tub... but that made absolutely no difference. Then one day I got fed up and dumped the water on her head, and said SIT DOWN!!! And she sat. She didn't stand up at all the rest of that bath. The next time, she stood again. So I tried again, I told her to sit down, and when she stood up again I dumped the water on her again, and she sat and stayed. Now, I just have to tell her to sit down while holding the big purple cup, and she will stay planted. lol. And, like I said before, I AM still able to use the same cup to rinse her hair and such. She knows I won't dump it on her if she's behaving. lol. Just make sure you are supporting her, because it does startle her and throws her off balance a bit.
I also think bathtime should be fun... My DD has her toys, and I let her 'swim' around and splash, but standing is a safety issue, which means it's a no-no.