I have removed the Apocalypse content pack from my game, however there still remains a now entirely useless sulfur item, sulfuric acid forge liquid, and several laboratory blocks with missing textures. Is there any way to remove these things from the game entirely?

What I did is go into the flans mod JAR file with winrar and delete the Apocalypse folder from the assets and com folders. Im sure that is not recommended but it seems to have removed the apocalypse dimension from the game (which is what I wanted). So it should work for you, but I would for sure back everything up first.

Did you just hack the jar or did you actually change the source and recompile?

Those are items from the apocalypse world which is part of the mod. These won't be removed.