Personally I would use a fine grit sandpaper to remove the paint. After the paint is off I would just work my way to finer grits of sandpaper and re-polish it. It is a little bit more work than some other alternatives but it will guarantee that the handle is not damaged and turn out an essentially brand new knife. The handle may be beyond repair but you could try some sort of chemical such as paint or lacquer thinner and some light scrubbing but it may damage the scales in which case you will be looking for a new handle.