can’t remember who taught me this one…grind down next to them with a hard sanding pad or grinder, overlapping circles all around the fin, pad/grinder wheel should be parallel to the bottom of the board (not parallel to the fin). when it’s all ground down snap the fin off from one side with one quick shot of the flat of your hand.
if the boards are not worth repairing this is definitely the easiest… also a good way to replace/repair a fin but you have to be careful not to go all the way thru the bottom glass.
The best way to to me is to use a 4’’ or 5’’ grinder with a coarse disc, like 36, just to keep the heat down, but you can do it with 60 or even 80 grit but it makes the job more time consuming. Start by knocking down the tabs in the front and back paying careful attention as to not grind into the fin. then take the grinder around the fin filets and through most of the roving, and then snap them off like a piece of ripe fruit.
Still Titus Kinkmaka had a faster way, back in the day, which though legendary isn’t the nicest way to do it; yet the guy has an amazing style. Yeah I’ve seen him in action, and Alikai laughing all the way.
if you have one lying around a dimond wheel for cutting concrete,tiles etc works really well for cutting fins off with the bonus that it won’t get smaller as you go if you are doing a lot of grinding