Temperance
Say it like you mean it? Be firm about it? Issue threats? Neither of us espouse violence but I have an extensive gun collection for you to choose from if it is necessary.
'Presumptuous' is a very nice way of putting it - I was expecting a lot worse, so thank you. I hope you do not take my words lightly; after all, if I have to kill him because you think I'm joking, or because you're being half-assed about this, this sin would be your burden to bear.