If you actually want to annoy them then here's how (had plenty of flamers add me after game):
Wait for someone who is typing in post game chat to add you.
Wait a few seconds so they're not ready to type, then add them.
Instantly call them "insecure", for some reason they can absolutely not stand that word and go extremely crazy.
Wait a few seconds for them to read and delete them before they can type a message back, they will start spam adding you out of fury and you should simply deny a few times and block them when you're done trolling them and wasting their time.
Worked 3/3 times in the last few weeks so far so if you want to waste their time so they cant queue up instantly again then enjoy :)