I think this is a male thing to do rather than female....not saying there aren't bunny boilers around but a lot of the girls I'm friends with have problems with an ex, albeit not on the scale of Sarky's nutter. The ex texting saying "i still want you back" when the girl is in a relationship, or even when the guy is in a relationship, is quite common, a lass on my course gets about 3 or 4 texts a day from her ex who has another girlfriend, all about the same thing "breaking up with u was the worst mistake of my life, love u babes, lets make it work". I don't get why these fellas don't have a bit of personal pride and leave it.
Maybe its a male thing on territory, if they've "been there before" they always think they can have a bit again. A girl I know is happily settled with her new boyfriend and is 8 mths pregnant, she ran into her old boyfriend in town and chatted to him for a bit, then he texted her that evening saying "don't u wish deep down it was my baby, we could still be great together if only ud give it a try". Talk about inappropriate!
As for advice for sarky, I think the best strategy is little red sox. Ignore any attempts from him to contact you. I bet after a few months of being ignored day in, day out, he will get fed up.