It was firstly requested on Steam. I tried for myself with the J.A.C.K. editor but failed to achieve this effect.
So here's the original request from PsyWarVeteran on the 14th August:
Half-Life - Making a Wall That Blocks Player But Lets Monsters Through?
Is it possible to make a wall that blocks the player but allows monsters to pass through?
I have been experimenting with "clip" texture but it seems to block the monster too - I though that was the case if the "monster clip" flag was ticked?Thanks