The player needs Conversion gel to put a portal on the right place. Water can wash the paint off the wall (to force the player doing something, don't want to spoil it..). If there is already a portal on the gel, water washes off the gel, but the portal stays on the non-portalable wall.

Is there any entity, which can remove that portal?

One way to remove portals is to create a thin, solid, nodraw-texture func_brush that is flush with the surface, which you enable (initially disabled) when you want to kill the portal.

You'll need a way to detect the water though, in order to trigger this properly. Also, could get really messy if there's a bunch of surfaces you need to check. Not aware of a better method at this point in time.