Personally, I'd keep it simple.

For say, water manipulation. Blood is usually something it can manipulate. In that case, sure you may WANT to learn it. but it should either be approved as okay by the GM, or you should know it's OP and not use it. If you're going to use it, then put limits on it. 

That's a problem we had in the Arena recently. A guy could, for all intents and purposes, say a word that had no requirement to hit the opponent. All it had the guy had to do was think the opponent was the target and say the word. Opponent didn't have to hear it, didn't even have to be within a set range, and they were screwed. For things like that, you need to have some sort of set up, or drastic effect. Usually, coming into contact with the person's blood, connecting to them somehow or something. Essentially establish a link.

Like in Naruto, that Immortal guy who acted as a voodoo doll, so long as he had the opponents blood. He had a pre-requisite to use his OP ability, and he had to fill it. Get their blood, make his circle thing and stay in his circle thing.