Yes, cut it back and let it go dormant.

I brought mine in when the temperatures dropped and some of the leaves were brown. I cut those off and it's now in a cool, dark room left unwatered - but shows no sign of going any more dormant. Now what?!

I move mine into my partially unheated garage. Sometimes my amaryllis will lose their leaves, sometimes they'll keep them. Whatever the case may be, wait until the leaves have turned completely yellow before trimming them off. This gives them sufficient time to absorb chlorophyll and any other reusable nutrients. If the leaves remain green, then just leave them. No harm done in just leaving them alone until next spring. However, if the leaves still remain viable for that long, and you do move them back outdoors, they'll still need proper time to readjust back into being back outdoors. Even if they were already used to being outdoors this past summer, they'll lose that tolerance over the winter being left in the dark the whole time.