What nutrients or medications would be helpful in stopping intrusive thoughts and memories?

It depends on the thoughts and if you have other psychiatric/psychological/stress issues that might be causing them. It would be best to consult a psychiatrist as only they can diagnose and prescribe for such conditions.

Antipsychotics are known to help some people with intrusive thoughts, but not all. There may also be other medications. As far as I am aware, no nutrients will have any effect.

I have experienced these for most of my life, and the only thing that I find that works is avoiding/minimizing stress, but that's not always possible.