Beezer= Ibiza
Diesel= I think you can work that one out
From the very ealy days of cupranet before the TDI tuning stuff really got going everyone else's username was cupra this and cupra that, but I'd done all the GTI tuning stuff years before hand and had got bored of it so decided to experiment with Diesel tuning. At the time Golf Diesels were crap, so when SEAT started putting the new fangled TDI motors in Ibizas in the late 90s I wasn't far behind in getting one.
The world has gone full circle though in the last 10 years or so.
Mind you I nearly always had an old GTI or Cupra of some sort knocking around in the back ground and had various other user names usually 'ghost' related because I've always worked shifts and was often the only person on a VW forum in the middle of the night! But in those days the forums weren't very stable and often the site would crash meaning you'd need to re-register and often someone else would nick your username if it was a good 'un. seatcupra.net must have had the most stable forum over the years to the username just stuck.