remove it from Blocks.class would be too much hasle, you could just hook the method that makes the block portal (i dont remember if its block obsidian or item flint and steel).
I have a old utility class that i done back in 1.3 that replaces classes at runtime (all of it, not just one), i could send for you to take a look, but it would be better to look if forge already provides hook for that, or if use asm, replace just the method itself, not the whole class