Dude, read the book. 18.1.a is pretty clear about the only time you can peal someone off.
I wouldn't necessarily call this clear regarding a finish line given the oddities of some of the rules:In rule 18, room is room for an inside boat to round or pass between an
outside boat and a mark or obstruction, including room to tack or gybe
when either is a normal part of the manoeuvre.
18.1 When This Rule Applies
Rule 18 applies when boats are about to round or pass a mark they are
required to leave on the same side, or an obstruction on the same side,
until they have passed it. However, it does not apply
(a) at a starting mark surrounded by navigable water or at its anchor
line from the time the boats are approaching them to start until
they have passed them,
So I would interpret this as you CAN force them above the line... you are also not sailing above your proper course if you do it because you are still sailing on a path to get you to the line in the least distance possible (heading any higher than the finish would be sailing above your "proper course"
