The following situation has happened to me several times:
A user in the gym is using some machine or equipment (a bench, a row machine, a bar, etc.). When I'm going to use another machine or equipment that it's apparently free, this user comes quickly and says "Sorry, I'm also using this machine". It usually happens when the user is doing some kind of combined exercises (for instance, doing a bench press and then immediately afterwards, using the cable row machine to "finish" their chest).
To me, the user isn't really using both machines, but instead, they are using one machine and "reserving" the use of another one (sometimes the user "marks" this machine as reserved by putting a barbell or a small towel over it), and I have the impression that this is not a legit action with regards to the use of a gym's piece of equipment, because one person can only use one machine at a time, and preventing other users to use other machines just because they are happening to be doing a combined exercise is not fair for other users, which don't deserve to wait. So in these situations I think I would perfectly be entitled to refuse to leave the "reserved" machine, remove the item set by the other user to mark the machine and, at most, I could offer the user to share the machine with me.
How could I explain my point to the other user in the least confrontational way?