I would try deleting those contacts manually in Spark first. In case they are indeed local copies.
Then you can try wiping out Spark profile for users, who have those contacts showing up. History will be lost (you can just move that profile to some place and then copy transcripts folder into the newly generated profile)
C:\Users\username\AppData\Roaming\Spark
The last thing i would do when nothing works, i would delete the user with defected contacts in the roster. But i'm not sure if you can do anything with the AD imported users. Maybe you can at least go into this user's Properties in the Admin Console, check its roster and try to delete contacts from there.
2.7.0 "nightly" builds (actually they are only built after changes to the source) are in my opinion more stable than 2.6.3 
It contains a few memory leak fixes. Of course, it may look and work a bit differently than 2.6.3 (new features, changes), so you should test it first.