This is really one of the most challenging aspects of micing a kit, and probably the most common "problem" you'll run in to ... but very easily preventable. I find that the problem really only comes in to play when you're dealing with an open hi-hat. It's just not that much of a problem if the drummer keeps it closed for the most part. A closed hi-hat is very rarely a problem. If it is, then you've got bigger issues.
If you just set the hi-hat up so that the most it will possibly open is like a millimeter, then that will take care of 90% of the problem -- right there. Only potential trouble is that the drummer probably won't like it, and will try to re-set it when you're not looking.
Try and explain to him/her that the hi-hat is not to be used as an extra crash or ride cymbal, and that there are reasons why those particular cymbals exist on a kit. Barring that, you can always resort to using a bottom mic, or even a side mic. Lots of people like the sound of mic'ing the shell better than the top anyway, so you might as well give it a try. It gives you more placement options, allowing you to either get it further away from the hat or more in the mic's rejection null.
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