Removing a sieve recipe from dirt, such as the following, does not actually do anything; the recipe is not removed, and there is no error.

Sieve.removeRecipe(<minecraft:melon_seeds>);

edit Gravel doesn't work either, however adding recipes seems to work fine.

you tried typing mods.exnihilo.Sieve.removeRecipe to remove it? as i have used it to remove a few from sieving dirt

It's a known issue. Seems like it's an issue on @Erasmus-Crowley's side.

Dangit, I was afraid of that. I'll try reporting it to Insaneau. http://bit.ly/1Ivivkd

This has been fixed in the code and should be fixed in the next version of modtweaker!

Open the issue if the bug still persists :)

Woot!

I can't reopen, but the wiki needs to be updated as the signature has changed and I no longer know how to call the methods.

It would also be nice if you exposed unregisterRewardFromAllBlocks and unregisterAllRewardsFromBlock, possibly as removeFromAll(outputstack) and removeAllFrom(inputstack).

Actually, I'll just open a new ticket to be sure.

I've addressed this in Ex Nihilo, however, some changes will be needed in ModTweaker to ensure this works.

See https://bitbucket.org/MikeLydeamore/ex-nihilo/commits/f2adf22f1cba71620462f7b7231a843e0a4c19ef for explicit details.
I have added brand new unregister recipes for most cases. If I've missed some let me know.