I’d try just pulling the string off. If that doesn’t work the cut it. There may be glue on the axle. Get a long thin tool that can reach inside and pick off as much as possible. If anything remains you can try to wear it down with some sort of coarse string or cord wrapped once around the axle. Make it long enough, say a foot on each free end, that you can hold it and pull the cord back and forth a few times. See if that smooths things out. Go back to trying to pick off more and rub with string as needed.
The cord/string warp is a recommended way to break in a wood fixed axle btw.