Fixed in the next patch. The exploit was that you could decalre war against weak players then force them to give you money for peace, repeat forever.
The fix is a fairly simple one. From now on the "to busy" check only happens if you are at war. If you aren't at war then you can contact them as many times as you want.
But you can't sue for surrender unless you are at war. Often, I wipe out a faction's cities, and then want to sue for surrender to obtain the sovereign....but he's always 'too busy'. Too busy getting his butt kicked....he should be willing to have an audience with his better.
In addition, when a sovereign surrenders to you, you get him added to your fold, though with a negative trait applied. This is a really cool feature, though left very incomplete.
If he's 'too busy', you are forced to kill him instead....and you don't get him added to the fold. But sovereign's can't die?!? I recommend him surrendering to you at the last minute...just like he 'escapes at the last minute', but he receives a different negative trait; something that better reflects getting stomped on.
Lastly, there are times when a faction gives up. All their cities are razed. He/she stands there doing nothing. This makes no sense. Imo, first off, the cities should not be razed, but should at the very least become neutral cities, comfortable with simply sustaining themselves. Second of all, the faction sovereign should be as obtainable as if he were a hired hero. There could be a tech for 'Sovereign hiring'. There could be a cost associated with hiring a sovereign. Most importantly, there should be a negative trait added that is reflective of the sovereign's new underwhelming status as mercenary.