Is there any way I can set pokertracker to only import hands for certain limits? I play mainly .25/.50 6max, but sometimes i donk around with friends at .10/.20 or .01/.02 NL and I don't want those hands in my database.

I don't think so - just don't turn PT on when you're donking and delete the HH afterward. Of course this could pose a problem if you're simultaneously donking with your friends and trying your hardest not to donk at $50 nl.

You can filter in preferences for it to show only certain limits. Just switch off the micro limits.

You can purge your sessions if you like.

This means erase individual sessions (ie 2c/4c nutbarring sessions).

Utilities > Database Maintenance and Options > select your session(s) and purge.