Your horse may not be doing this for fun. He may be doing it because he does not tolerate heat well and he is trying to cool off.
At least, that is what my 27 year old has done all his life and I would not ask him to stop. I can tell that he is actually cooling off because he does not “play” in the water on a cool/overcast day. But when the sun comes out, he needs the water to keep himself cool.
He and his BFF have a big Rubbermaid tub with an automatic refill float ($12.95) attached (over the back side, so he cannot hurt himself on it.) [NB: the tub must be tilted back so the float stops the water before it overflows from the front.]
Yes, the water can look nasty (I empty and scrub 2x/week; it refills on its own) but dirt sinks to the bottom, so the horses have a continuous supply of water no matter how many times they ‘take a bath’.
Since the float stops the water from running as soon as it reaches the bottom of the float, I leave the hydrant on all the time.