Camping is boring no end...
If its so dang important to restrict someone from doing the quest over and over and over and...
well put a personal timer on the quest then for -None-'s sake IE you finish the quest, you get a timer set that makes you unable to do the quest again for x amount of time, the NPC stills stands there but he tells you to "Sod off" until your *personal* timer resets, that can't be hard to implement can it?
I have never done the Dodga quest because I can't be arsed to sit around staring at the sky waiting for him to spawn... its boring no end.
Maybe that is what you want us to be; Bored from missions, bored to tears from running around the land in vain looking for something to kill, bored from camping a static NPC for a quest?
Some people might just get to bored to actually bother to pay for more boring stuff the next month (not a threat... just a reminder of how the human brain works)