With what and where are you micing the snare? A different mic or positioning might help significantly.

Is your room treated? If not, that's a good place to start. Lighter hats might help too.

One suggestion I've seen but haven't tried is to add a hi-hat mic if you don't have one already, and have it play loudly into the drummer's headphones.  That should tame his hi-hat work.

But the best advice might be to embrace the bleed. You're not going to get 100% isolation on percussion instruments sitting a foot or two apart.

• Hypercardioid snare mic. senn MD441 is good. Beyer M201 is also hypercardioid and very affordable (under 200 euro) but I'm not familiar with it. I've read good thigs about it and plan to pick one up soon.
• Use a snare mic that makes any hat bleed usable. The mics mentioned above are dynamics, but the hi-hat bleed will sound better than with a 57.
• Don't try to eq out the hi hat. That will usually makes things worse because you are left with even worse sounding bleed. (although I know an engineer who's using the snare mic with lot's of eq and compression for attack only with good results)

If that's not enough there's something wrong with the balance of your drummer. Smaller hats may work unless he's not bashing them even harder.

Hope this helps.

Peter