unfortunately, its normally (based on the one i've done this to and the other two crappy cranksets laying around in my room) the middle or granny ring thats structurally part of the crankarm, so you can do it and ride a 24/26 or 34/36/39 whatever ring with them, usually. drill press works well too. you can also use a hacksaw(real tedious)/angle grinder/dremel(use a million cutting discs) and cut through the most narrow bit of each ring except for the one you want to keep......the only cranks i've seen that you can't do something similar wtih are some old plastic-coated-steel, pathetic shimano guys.