If you mean mirros created using CreateMirror() then you can probably make a plane under it, flip it, put it a small distance under the mirror, and you shouldnt see it.
If thats not what you mean, have a look at stencil mirrors by Tom: http://www.blitzbasic.com/Community/posts.php?topic=40770
If thats not what you mean, have a look at stencil mirrors by Tom: http://www.blitzbasic.com/Community/posts.php?topic=40770