Ignore it. Minor mushrooming shouldn't affect your shot, so if you can ignore it completely then you'll be fine. Easier said than done for most.
Fresh (i.e. sharp!) razor blade/exacto knife. Just trim it off like you're peeling an apple. Then use a tip tool or sandpaper to dome it.
Coarse-grit sandpaper. Fold it up or tape it to a block (or use a sander like the other guy linked). I press the tip against the sandpaper with my thumb and spin the shaft with my hands to grind it off. Be extra careful not to scratch the heck out of your ferrule tho. I use the scuffer on my It's George tool for this purpose to good effect—good tip tool btw, would recommend.
Mushroom grazer. Never used one, but I assume that it would work well given its reputation. I try to avoid unitaskers, but it's cheap enough.
Have it done on a lathe. Should take less than 5 minutes. If you were a regular at my pool hall, I wouldn't charge for this. Otherwise maybe $5.

Edit: I totally forgot to mention that emery boards work perfectly for this procedure and are actually my preferred tool.

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