How can I decorate or hide a mirror too expensive to remove?

Hi ! Perfect solution ... removable wallpaper !!! There are so many styles plain , traditional , fun , themed to quirky and funky. Check out websites like :

Wallsneedlove.com & tempaper.com

You could use tape to create a design and paint the exposed areas.

If wallpaper is too costly, use fabric. If there is crown & base molding on the wall, you could simply remove them,, staple a flat king sized sheet or fabric (patterned is best) directly to the wall top and bottom and replace the moldings. Or staple pleated or gathered sheets in your chosen pattern to thin wooden strips and nail or glue the strips to upper and lower edges of the wall.

Depending where the wall is....hang a drapery rod and hang drapes, treat it as a fake window and keep drapes closed

If you want to remove the mirrors be very CAREFUL. if they are mirror tiles try using a heat gun available at Harbor Freight for cheap. when the adhesive becomes warm just pry them off. If is big sections of mirror, use clear contact-paper over the entire section you want removed, wear safety-goggles, major gloves and be careful, cracked glass is just like a gillottine and can cut of body parts!! you do not want sections you can not handle to all start falling at once so plan accordingly. also protect your floors for easy clean up