You'll have to sand it all back to white wood, or maybe try bleaching it.
All depends on how old the bat is as the sun bleaches the sticker shape/ pattern in to the willow. The older it is the harder it is to get out.
ive had some bats before that have been like 6-7 years old and only way is to use 80 grit keep sanding and sanding and sanding! eventually it comes out but its a ball ache