Re: "Beware the generosity of Kings" - A warning
Why is the king threatened by them? Is their a story behind this? This will help determine what he should do. With the little information you have given I would recommend rather than "Out of sight, out of mind." I am reminded of the great Nicollo Machiavelli. "Keep your friends close. Keep your enemies closer." What he should do (i am assuming he is evil and not petty) is he must keep his secrets away from their do gooder eyes.) Enchant them into working with him. This doesn't have to be magical. He could get noblewomen of his court to seduce them and getting them entrenched in a life of adventure that benefits their family.
If he is just shortsighted and petty send them off. I say if shortsighted because the kingdom obviously needs heroes of their caliber present since the menaces they defeated were there and something like that may return.
If he is petty but not an idiot have him create a title such as "Heroes of the Kingdom" this title involves a ceremony which the party pledges it's allegiance to the throne and places a mind control spell on them forcing them into obedience. The king may never do anything but he will feel better knowing they are enchanted.
Either way it becomes a plot point. An interesting political game will ensue especially if he isn't evil. How will the court respond to the "evil" act of enchanting heroes. Some will understand and some won't. If you are war oriented it could rip the kingdom apart and create civil war. You could make it more interesting by having the characters side with the king because it is a group of self serving (Evil?) wannabe tyrants vs. a small man of a king. Heck the PC's might try to take the kingdom for themselves. Even if it is with good intentions they prove the kings fears right.