Never. Either warn Alice, ban Alice, or leave it alone. Attempting to give Alice an Internet restraining order is only going to cause her to find more roundabout ways of expressing her views (e.g. dragging the conflict into other threads.) Just keep it simple.
Even more simple: If Bob doesn’t like Alice’s reply, then Bob should downvote Alice’s reply.
Even more simple: If Bob doesn’t like Alice’s reply, then Bob should downvote Alice’s reply.