Unlike child support, spousal support is often very difficult (if not impossible) to modify after the divorce is final, so getting it right the first time is a must!
The problem is, that there were arrears. And some states add on interest. And, to be honest, I agree with this. When one doesn't pay their child support, they are in essence forcing the CP to cover their portion of the child's expenses. Basically, the ncp forced the cp to give them a loan.
And as you have found out, they will continue to collect arrears and interest on child support long after the children have been emancipated. And apparently, spousal support as well.
You may be able to get it reduced if you get a good lawyer. You could always offer your ex a settlement for less than what you want, this would mean that you would have to give your ex a lump sum.
Of course, you could always contact your ex and see if your ex is willing to forgive the outstanding balance.