Grenade launchers still get me sometimes, but I've learned a bit on how to avoid.
I normally don't like to give hints on the game or my playing style, but this I don't mind. If I can help other people die less from a weapon I dislike, hopefully people 'toobing' will use something else when they find their attack less effective.
I always used to play with music in the background. Now I never do. There's all kinds of sounds in this game that when you hear them, you know what's being used where (in conjunction with your radar sometimes) and how to find the person/weapon you're looking for. Or avoid. So when you hear the click/whoosh sound of the grenade being launched, just strafe to the side. Run if you can/have time. Try not to be right against a wall. Even if you don't know who has the gun and where they are.
Like snipers, toobers typically will camp and find one crazy angle that looks in on a busy spot, like the top of the catwalk in the Facility. So even if you don't know where the person is but you hear the sound, look at your radar, and the red dot that appears might earn you a kill, not the toobin' noob.