As far as I know, you could either:
- Do what ISun said.
- Use MCEdit to delete said block.
(I think as long as you don't touch much else, your mod stuff shouldn't randomly turn to vanilla stuff like it used to do, make a backup anyways, just in case.)
- Disable whatever mod said block is from and load the world, this will cause you to lose everything from that mode within any loaded chunks unfortunately, but better that losing your world.
(Plus any items you had you can probably cheat back if you really wanted to.)