Your best answer is to teach them to stay out of there and leave those alone, removing them or covering up the faucets, doesn't solve anything, but if you teach them this is not acceptable, and discipline them it only takes a couple of days and you got the problem solved. Here's how.
First you take them into the bathroom and tell them this is what you expect from them, (you may think they don't understand, but if they can turn on a faucet then can understand,) you tell them to not turn on the faucet or you will put them in time-out for 5 minutes, (or course I never used time outs) If they go back to it again then you give them a swat or two, if they go back again the swat get harder each time, I usually swatter hard enough the first time, I didn't have to keep doing it over and over, lots of time the problem is the one giving the swats, is not consistant with the discipline, so the kids learn mom doesn't mean what she says, so mom or dad teaches the kids they can get by with it one time and maybe next time they might get up and discipline them. (child is confused) A child needs to know they can depend on the parents being a person of their word and they in turn will learn to be a child that is content with the rules and they feel safe and protected. A child wants to know where the bounderies are, but we as parents fail when we aren't consistent to enforce the bounderies. I hope this helps you with your boys, and I know it will help you both the older they get, because it makes discipline so much easier when they learn it at a young age, and they know when they are 10, 13, 16, that you mean what you say.
my son & daughter are 39 & 42 and they know they can still trust my words and they trust me with my grandchildren also.