If you are sued on a debt that you've already settled, then whoever sued you will have very serious issues.
In the past we've handled several of these types of cases, and the amount of money that the debt collector has had to pay for coming after our client over a settled debt is incredibly high.
So you can settle a debt collection lawsuit for money (although usually there are better options) and not worry about being sued again by the collector on the same debt.
Most of these companies have decided they don't want to pay the high price they have to pay for that type of non sense.
Hope this is helpful to you.