It's called muscle control/good form. I had to work with a client for ages to stop his habit of raising his butt off the ground. DRIVE your feet into the ground with your hamstrings and OPEN up your hip flexors. that'll keep your arch AND keep your butt down.
Your butt's coming up because you're telling it to, not because of where you're putting your legs or that your heels are on the ground. Keep it on the bench. Concentrate on it. Drive your ass and your traps into the bench at the same time.