Back in season 2-3 I was quite a toxic player (Real life issues, took out anger on game). So I know first hand what goes through the mind of a toxic player. There are really two directions you can go
1: Ignore the player: What does a toxic player want? For you to respond. He/She loves it when you try to respond, it only adds fuel to the fire.
2: When the toxic player says "You so fucking bad, stay under tower dumbass" , just say "Ok, thank you for the input". A toxic player many times doesn't believe they are in the wrong, but when you respond with kindness they start to realize "O crap im just being a jerk", although this doesn't work with everyone (Because some people like being jerks), it does work sometimes.