[Imp]

Simon why not add something to your house rules, I have no problem with losing a VH at the end if its being contested.
The silly one is players trying to sneak a lone size zero unit on meaning you use silly tactics like puting your defenders right on it or calling in your arty on what you own for the last couple of turns.
Simple rule that size zero cant take in last 2 turns would work, if you like unless supported by other units within visibility & range of the VH.
Can check the map easy enough at game end.
Other option of course is dont play them.

The good news is bigger picture its a bad tactic normaly used by people losing badly. That unit near a VH has extended the game or at least stopped it finnishing early letting you chew up whats left for longer if no other VHs are being contested.