If your swingarm is like my '99s, this is what you're up against.
If you can get a small screw driver tip in against the lip of the rivnut, chances are you can get the bolt out if it's not bunged up. (chances are the reason it's spinning is it's been over tightened.)
If you can get the bolt out, you can then drill the top portion off the sert and it will fall into the SA and you can take it out by removing your wheel and chain adjuster.
Replacing it is as simple as finding someone with some metric nutserts and a tool to install it.