See:
http://www.gammon.com.au/mushclient/mw_other.htm#DrawingOrder
No, you can't remove a single text rectangle, short of drawing something you happened to know was underneath it back again.
The drawing is quite fast, I have found with something like the mapper, that simply drawing the entire window with your background colour and then redrawing everything, works acceptably fast.
But if you mean, you want an elaborate window with some text in the middle, that might change, just store the text rectangle position, and clear it whenever you want to remove it (by clear, I mean draw the underlying background colour on top of it).