It's possible to completely remove a permablock from the play area without permatizing another block, and it's possible to leave no permanents on the play area even after having permatized a block. If you use the hourglass to permatize a regular push block, and multipush it off the map in level 1, you can get rid of it without having to permatize another block, cluttering the play area.
I'm pretty sure this doesn't remove the block, but instead just pushes it off the viewable board. I can tell this because if you push an hourglass into the space where the block WOULD have gone, you get the permatize particles, the hourglass shatters, and any original permablock supported by the hourglass gets unpermanented
Edit: just realized you can also permatize a covemonty and kill them to remove clutter
