Lots of solvents will remove it. Start weak and move up the list until something works. I believe we used Simple Green.
Still, you will have a ghost image to deal with. As Maerin mentioned, you can wet sand (If you try and wet sand, use paper in the 1000 grit range, then 1500, then maybe 2000. Then polishing compound and then wax. It is pretty tedious.), OR have the entire transom "wrapped" with vinyl that matches the color of the boat as the background for the new name graphic. We did not opt for that since I was comfortable wet sanding, but the thought crossed our mind. It does open up possibilities for a more fancy graphic that covers the entire transom should you go that route.
2000 Navigator 4200 Classic
(NOT a trawler)