What is the exact word for ...

[hermit]

one response that comes to mind is "rubbing it in".

for instance, when somebody always reminds another of a debt, the response might
be: "hey, don't rub it in!". (perhaps referring to "rubbing salt into a wound")

hermit

[ETutor]

This is a tricky one, without knowing the context of what you want to say. Here are two phrases you could use:

I did him a favor some time ago, so I often remind him that he is indebted to me.

I owe him greatly for his help. And every time I see him, he brings it up and hangs it over my head.

I hope this proves useful to you!