Rehab contractor botched stairs and isn't replying. Any tips?

[Dean Attali]

I hired out a GC a few months ago to do a major rehab on a rental house. There have been many issues that the new tenant has discovered that made me lose confidence in the contractor, but the latest one scares me because of liability.

3 weeks ago they told me that the stairs have nails sticking out of them and that they can't hammer them back in. I've been trying to reach the contractor ever since and he isn't responding. This week the tenant also told me that they they slipped on the stairs and the nosing came off way too easily, and that after that happened he took measurements and noticed that the stairs were not done up to code. In my contract with the contractor, it says I have a one year warranty on any defected installations. 

What can I do in such a case that he just isn't responding? Since this is now becoming a real liability issue, can I consider suing if he doesn't fix it, since I'm the one who can get sued by the tenant?