Contractor/subcontractor problems

We undertook a renovation 18 months ago, and moved in (and had mostly finished) in September 2012.  Our HVAC sub has since disappeared – initialy due to Sandy, but we have had almost no communication with him since November and he has not been in touch with our contractor either, although he sent a few nasty emails to the contractor.  There were three areas of work that he did not complete, and one area which was done sloppily and which the contractor had to complete for him. We’ve hired other people to finish two of the other items, but there is still one outstanding piece of work, as well as warranty and maintenance issues on equpiment that need to be addressed and as-built drawings which he promised us.  He is owed $4000 by the contractor, although once the additional work that the contractor did and the payments we made to other people to finish his work have been deducted, the balance owed to him is actually negative.  We have not made a final payment to the contractor, because we feel that we need resolution of this issue and final sign off from the sub before we can do that, but the sub has been nasty and threatened to put a lien on our property (this was before all communication ceased).  We would like to conclude our contractual obligation with the GC and get these issues resolved with the sub once and for all, but are really not sure what options we have to do so, aside from just gambling that he won’t come back to haunt us in the future.  Has anyone had experience with a situation like this or have any advice about to resolve things under these circumstances?  Thanks for any and all thoughts/help/advice!