RedDash16

Or make an impasable sector

UDMF map format has a "block everything" property. It doesn't block explosions, though. A rocket to one side still damages the other side. I think a transparent 3d floor could fix that.

Or a ceiling lowered to the floor, with untextured walls. Pretty sure there's some way to make them in the traditional engine, TNT.wad has invisible walls in some levels.

The ceiling option would also block line of sight.