The staple of all TDs. (This isn't actually for a TD, though) Creeps that should ignore a specific unit type unless it's blocking them entirely.
The unit type in question is a czech hedgehog, which should be used to funnel enemies but not to block off their path entirely - and it should not be possible to distract the creeps with an endless row of hedgehogs along the road, which they attack one by one until they finally arrive at their goal 24032 minutes later.
lol i acctually thought a czech hedgehog is an animal for a moment :p
well i have an idea about how to check whether a path is completely blocked or not. its kinda lame though so i guess other ppl can provide better answers. this is my idea anyway: whenever you order a unit to move somewhere, create an invisible (no model) unit at that location with infinite speed that cant attack and tell it to move to the target location. then wait 0.1 seconds and check if it have arrived. if a path exists then it should be there. if its not arrived the path is blocked. then destroy that unit.
That's what I thought too. But you might want to go with a more classic (SCI) approach. Don't ask what the approach is, but I know it worked in SCI with the invincible anti-wall units..
However, this solution would require a lot of units, so I suggest having only a single "path finder" that constantly runs through the maze. This path blocker would have collision with your "defence", but not with the creeps running the maze. It could even be used to find the optimal path for smart-creeps o.O
It's like quantum physics: the pathfinder takes every single possible path at once, then arrives at the destination using the most efficient one.
the unit you're using to check for a wall will usually go idle somewhere close to the region it is trying to move to. I would suggest to attach a large region to the invisible unit and kill all of the units that are the walling units in that area belonging to the player. works very effectively on making players both upset and helping them understand that they aren't able to wall on the map.