"u smelly breathed twat ... &c."
The best way to reply to messages of this kind is either:
a) Ignore them
b) Reply to them in a tone of overweening superiority and condescension, thus:
Thank you for your opinions, which have been laughed at and subsequently discarded.
In answer to the points you raise:
1) Is that supposed to be an insult?
2) Your wish will eventually be granted as all people die, even you;
3) No. You aren't;
4) All road-kill is wrinkly, therefore this word is unnecesary to this, your most finely-honed thought;
5) Your final sentence should read "I know where you keep your horse and I am going to shave it bald and cut its ears off". Spelling aside, I would personally pay a very high price to see you attempt this feat. I'm sure the horse will be entertained for hours as you pursue it across its field in your futile attempts at shavery. Good luck, dimwit.
6) You're welcome.
Clearly this kind of reply will likely solicit an answer. My preferred method of dealing with said answers is simply to ignore them as this will confuse and annoy the perpetrator who, on the one hand has received a reply challenging him to do better, and on the other receives no acknowledgement whatsoever for his efforts. You could call it "training for real life" if you liked.