I'd be very careful using anything to remove the signature because you might ruin the finish on the guitar. If the autograph is anyone known, you might be ruining a collectible. It might be best to call a guitar maker or luthier to find out what would be best without ruining the finish.
Login using the form on the top of the page to post feedback (if you are a registered user). If you have not yet registered, click here to do so. It's FREE!.