the type of quests you want should be configurable in config file, and when you click on "refuse" the NPC should have a random chance to switch to another quest.
I'm not giving any mithril or gems or silver to greedy NPCs...
quest_bring_basics=20% quest_bring_valuables=20% quest_kill_player=20% quest_kill_NPCs=40% (or simply the remaining percentage of the previous)
could be in the config file.