First, no one claimed that imprisoning them could be done with ease.
Second, were they as powerful as you imagine, they would have responded to the outcome of the contest by slaughtering the entire military &c. What you seem to have missed is that they were only able to deal the final blows to the Goblin King by virtue of the efforts of those nearly killed earlier in the battle. When the full might of the restored adventurers was lined-up against the sociopathic party, that party had no choice but to back-down.
while I do agree with you about it being bad writing (except on the dropping it for a plot hole that most Marvel and DC comics have as well) I will also say I wouldn't be against him returning rather than being in jail or dead just so that we can eventually see the old man or hopefully the healer finally punch him in his smug face their selves. This antagonist is just too aggravating to allow him to be removed offscreen. Plus I feel exile would be worst for this narcissist; just thrown away and ignored as even the Kingdom doesn't see you as worth it.
Besides Exile back in the day was more dangerous than expected. If word spreads fast enough I could see him basically not allowed work in any major city as everyone waits for him to die on the side of the road after the loss of all credibility.
I guess it depends on how the kingdom sees attacking other adventurers out in the field and if this isn't something that gets the death penalty, then they also have to explain why his guards weren't chomping at the bits to kill him instead of exile. Also could see him still having enough support with some of the more uncompassionate elite of the this kingdom who see nothing wrong with this mindset of using the weaker commoners for your benefit.
Its fine if you want to drop this series but I will keep reading for a while to see where this goes. Besides I have seen so much worse plot holes in way better series that I can deal with this one.