You cannot delete old questions you've asked on here
The most you can do is make the question private or maybe go to "Contact Us" at the bottom of the screen and try asking them to remove it, but chances are they won't unless it violates the terms and services
If I really need to delete question, I will report it. Sometimes I report my own question if I can't find any other way to delete it. I usually chase after moderators: adding them as friends so I can complain to them about it.
I don't know how often they respond though. They are probably very busy people.
To delete your own old questions -- if you're still receiving messages about it and it's bugging you -- you can send the link to GAG Admin (the first friend on your GAG Friends list). Otherwise, having the old question up is often helpful to others who are looking for advice on that or a similar issue...