Yeah, there really is no real way to prevent portals from sticking. Mytown works to an extent albeit in a ham fisted way by disallowing the use any mouse buttons while in a claim. But the thing about MyTown is that it can be easily skirted around.
For instance the Wand of frost, you used to be able to use that in claims to break peoples leaves and glass if you arced it just right. As MyTowns protections prevent you from clicking your mouse while looking at any block within the claim all you need to do is aim at the sky.
Even with mytown if you are just far enough you possibly might be able to still shoot a portal in. Unless they have added a permission node that disallows ALL portals.
TBH my suggestion would be a 2 pronged attack. I'd use MyTown and I'd also utilize the inbuilt portal contact whitelist (if it still works) that only lets certain blocks to form a portal. You could set it so only concrete and whitestone can take portal (or whatever blocks you think would work). Then you can let the player decide whether they want portals in their base, this also allows for player made test chambers, so more potential for fun.
