Never tell people youre in promos for one, cause that will trigger some piece of shit to go full troll on you instantly. I dont know if you said it before they made threats, but dont say youre in promos after they make threats either. If they were joking, maybe they wont grief now, but if they smell blood, they may flip from trolling as a threat to actually griefing.
Never beg, that just makes them get a boner the more attention you give them.
When this happens, take a screen shot (alt+printscreen), past it into paint afterwards, submit a ticket. Dont tell them youre reporting, but you can defuse their inner piece of shit by just telling them "Do whatever, all you can do is slow me down, you wont be the first piece of human garbage Ive stepped over this week." Then say nothing else, theres a chance they will forget, or start to play legit if they cant get any more pleasure from noticeably annoying you.
If they persist in trolling, like smiting your camps, go to the replay, and clip out a video of them following you stealing your camps. Submit this video along with the screen shot. This will not just be some "salty kid report, add it to the pile" no, this will show someone who handles reports that THIS player IS a PROBLEM. This evidence will go a long way to give any context to future reports where people comment similar things.
Then save a favorites in your browser on their OP.GG stats page and check it from week to week... Take some satisfaction when you see the match history come to a total halt... that was their ban.