1. If your crank arm is elliptical and keeps coming loose, replacing it with bolts will hold the crank arm in place for a while, but it will eventually come loose. Should this occur, you will require a new crank arm. When the square taper becomes elliptical due to wear, it will strip the bolts.
2. Should your crank arm bolt come loose, this is typically due to the thread on the bottom bracket spindle rather than the crank itself. In such instances, you will require a new bottom bracket spindle.