All that being said... a sin doesn't "unsave" a man. I don't believe for a second that David ever lost his "salvation"... and he did some heavy duty sinning. That being said... had he hardened his heart and ignored the warning from the prophet... I believe he too could have lost the Spirit of God just as Saul did.
But the answer is yes... someone can turn from God and return back. Remember the prodigal son was alive and left... dead while gone... alive again. Now... I asked folks this before and no one answered back. If you can show me anywhere in Scripture that alludes to the idea that a spiritually dead man is still a saved man... then you're on to something. If you can't... then I can't buy it.
Folks tend (on the NOSAS side I'll pick on) to forget that even when a brother is caught up in sinfulness... that doesn't automatically negate his being saved. If the brother refuses correction and tells everyone, God included" to pretty much pound sand because they right like what they are doing... that person is on a gnarly road. But nevertheless... God is times more patient than any one of us and patient even when we're living like a moron.