Delete, reinvent, abandon, kill off for the plot, etc. Whatever eases your heart. If I dislike a Sim I made, it is generally because I designed them to be dislikable (having to love everything about them can get pretty boring). However, I do understand what you mean. Sometimes they morph a bit.
I once designed a couple to have a vampire daughter from an affair/ex and I planned to make the toddler practically the Omen kid and her step father miserable. He turned out to be a great father and all around great Sim (same for her). I never expected to get attached to either. Funny how it works out like that.
As for Melinda in particular, I do not often feel the compulsion prematurely end my Sim creations, but I am not particularly merciful either. Especially now that you can shift click with testing cheats and have death by rabbit. Too easy to not be a viable option. So you playing her as long as you have is far more than I would have. At bare minimum I would break them up and focus on her husband's new life. If you still have hope for Melinda, reinvent her and see how a new path treats her. The kid would be wherever I am actively playing/want him.
