Before your husband throws their letter away, he should read it, just incase it contains something important that you do need to know. He doesn't have to tell you what they wrote if it's unimportant and will cause you stress. That's what I would do anyway.
I think this too.
Personally I can't see anyone going to such lengths to speak to you in person, to just want to have a relationship with you or dredge up ages old history that you clearly have no interest in bringing up.. you made your feelings clear and they can't force a relationship on you if you don't want it, and you do have the power to control that, so there could be a genuine reason for them contacting you in this way.
Whatever it was they were so keen to discuss may have some bearing on your health or your future in some way, or even your children, so I'd definitely get DH to skim it, if there is nothing in there that will effect you or your family in any way then just bin it and you'll never have to think about them again.
But the other thing to consider is that you will also have their return contact details, so if there are any problems going forward you would be able to instruct a solicitor to stop them contacting you, I'm not sure it is something the police will really get involved with but that doesn't mean you don't have legal channels available to you.
Hopefully that is the end of it though OP.