How do I stop objects from enterin but allow others to pass.

Simple concept really,

I want the player to be able to go into a lake for example but i want to prevent monsters from going into the water

however i cant figure it out... i dont think a solid toggle would work i feel like it would get everyone stuck.

Just a thought:

Invisible walls on the 2 sides of the lake (not solids). Check if monsters collide with the walls and move them to opposite direction.