I would advise the use of anyone of the cleansing or protection spells/charms that have been mentioned previously.
There is the option that you could try and close this doorway. Mirrors are naturally a window between this plane, and the others, they need only be opened. But like ever door or window, they can always be shut. If you have consecrated water, I suggest you write a chant of protection, then using the water make marks of protections (these could be anything from crosses, to stars, really anything that is related to protection) upon the surface of the mirror while chanting the words you wrote for this purpose.
It is possible that you would be subject to sometype of attack during this process, and I suggest you wear as many protective charms (jewlery, clothes, scents, etc.) as possible to help ward away these counterstrikes.
If this does not work, then take a step back from the situation, reevaluate the factors, find out what is really going on, and then look for other means to the desired outcome.