Terraria – Can you hide all transformation items

You can Alt-Tab out of the window, or simply click out of it if playing in windowed mode, and it will pause gameplay. It's hackish, but it does pause (you can also do this during loading, and it will keep loading).

As James has pointed out, this is single-player only. There is currently (and I doubt there ever will be) no multiplayer pause.

You shouldn't use it as a building material because the presence of a lot of Ebonstone causes the sky and music transition seen in the Corruption, and allows Corrupted enemies to spawn.

What you could do with it:

• Place it in your inventory's trash slot to get rid of it; use Shift-Click for maximum efficiency.
• Place it, then purify it to get more Stone, if you happen to be short on that (which will never happen because half the map is Stone).
• Store it on the assumption that it does something useful at some point in the future.
• Use it to lock a passage to players who don't have a Nightmare Pickaxe or higher yet. Then again, other materials, such as Obsidian or Hellstone, would be better suited for such purposes.
• Use it to build a Corrupted enemy grinder for infinite Rotten Chunks.

As of patch 1.1, there are also a few more "real" uses:

• You can combine it with Demonite Ore to make Demonite Bricks, which are a feasible building material.
• After unlocking hardmode, the Corruption can spread via Stone Blocks as well, so you can get an Underground Corruption going by planting a few Ebonstone Blocks, then waiting for them to spread.