You should be able to cut it up with any editor with a visual window. How can you hear it repeating? Is there a click, or a silence?
If there's a click, its because the end amplitude of the sample does not match the start amplitude of the sample. To sort that out, fade the first few milliseconds of the sound in from 0%-100% and fade the last few milliseconds out from 100%-0%. That'll make it seemless. I stress 10ms tops really. No more than that, and you'll also need to cut the sound exactly where the loop part finishes and where it should begin again.
If there's a silence, then that's because of the MP3 padding, and you'll need to remove that with some sort of save option. You can do all this with cooledit pro, but basically trim it exactly, fade in/out the first and last 10ms of the track, and save the MP3 without any ISO etc. padding.