Just add a very tight working radius, even though it would make using them more tedious, it would fix this problem slightly, making it annoying to eat a whole planet with it.
Or add a delay between uses that builds up as you increase brush size, it should keep counting though even if you left the build block, to prevent abuse. Fixing the brush cheat would be necessary first though.
Or perhaps only make blocks that were placed by the player in the first place remove-able, though this would hurt the build block\'s functionality a lot.