If they have more than one provence, Offer them peace, then demand tribute, grab all the provences you can up to about 10% less than your victory total (which will be at 100% if you control all their provences.) Then you will own all those provences and have a truce with them for 5 years. after 5 years declare war again repeat as needed, till you declare war against them while they only have one provence. After you win that war when you offer peace a new option to annex will appear, select that option and your hated enemy is no more (at least until the rebels revolt and form the nation again 
) Have fun!
) Have fun!