Instead of having a boss that drops decent value loot to whoever kills it on death introduce a bounty quest system into the game. This would work by giving each major town a magistrate/lord/jarl/sherrif npc that you can visit that will give you a bounty quest to hunt down "the local bad guy in his, her or it's lair". This quest should also be able to be completed retroactively so if you kill a delve boss and then visit the "magistrate of Vulkhel guard" you can complete the quest and gain the experience and a reward.

Obviously there are things to be worked out like maybe not providing a compass point for the quest as that may spoil people's feeling of discovery etc (maybe rough directions on a "drawn" map instead or something?) and how to handle the set piece items that genuine players try and acquire from these bosses but in one fell swoop you completely remove the principle stable method of gold farming with bots and potentially add something in that makes delve bosses a little more characterful.