Are there any rules for denying PCS leave?

Technically... your command does not have to release you until the last minute. If you have been authorized 4 travel days , then they do not have to release you until 4 days prior to your RNLTD. As the others have said, leave is a privilege and leadership can deny it for just cause.

Coming back from overseas the last time... hubby put in a request for 30 days, he was given 21 days. (he had 70 days of leave accrued at the time) The base was going to be finishing up an inspection and they wanted him there for the entire thing, not leaving 1/2 way through it and not outprocessing during it. Not happy, but the AF is in charge!

Taking leave is a privilege. It can be denied at any time at the commander's discretion. But if someone's getting denied PCS, they dun f'd up big time.

What kind of leave? Actual PCSing? Or House Hunting? It can depend on the command the AD member is with and what is happening at the time.

Leave is a privilege and is not guaranteed!