Don't offer them anything else
At the start of the quest, offer them a really small reward. When they ask for more, the character can explain it's impossible.
When they look for some other quests, they find there aren't any.
Teach them that asking for too much much ruins relations with towns and groups and if they don't take the quests being offered, there is nothing else for them to do.
Steal the gold
One day they wake up and it's all gone. Give them a quest to recover (part) of it.