Make a actor slide along a blocking volume?

My player is just a camera with a sphere at its root. My level has a blocking volume around the sides of it to stop the player moving the camera outside the level, like an RTS camera for example. I’ve set the blocking volume to block the camera’s sphere. This all works correctly except for one annoying problem. When the “invisible” sphere collides with the blocking volume, it just stops dead. This is very jarring for the player. Instead I would like the sphere to slide along the wall of the blocking volume. That way when the camera is moved to the edge of the level on an angle for example, it will just smoothly slide forward along the invisible wall instead of stopping.

I understand that physics can solve this problem if you want full blown physics. However you can’t use physics for a camera controlled by the player as it just falls to the ground and rolls around the place.