Brother, if I saw a towel on a bench, and the person was off doing another exercise, then I would simply take it an hang it somewhere. Just because they are training someone, that doesn't make it right. I wouldn't pull that shit, and neiher should they. If they want to do a circuit session, they should do it when the place insn't packed, or use equipment that there is more than one or two of.
Some guy would do that at my gym. I called him Hollywood. He would always talk on the cell phone real loud how he is the star football player at his high school. He weighed about a buck 50, wore those horrible and tacky 'shorts', that really look like girly man capri pants, and an upside down visor. He would leave his cell phone on a piece of equipment and walk away to chat, or to use other things. If you moved it, he would get pissy. One day I needed something he was using, so I simply tossed his phone on the floor. He tried to tell me he was going back to that chin bar in a minute or two, but ignored him, as I did a few more reps with 50 wrapped around my waist.
Sometimes rude fucks just need to be put in their place. Try it next time. It's better than getting angry, or even getting nasty with the person. Just make it obvious they are rude dicks, and then ignore them.