First, you want to check if they are retainer or contingency. You want to work with retainer only.
Next, tell them you will call them back.
Do call them back when you are at a comfortable place to talk to them. Let them tell you enough about the job so you can be sure it’s not for you BUT so you know enough to be able to refer a name or a source for a name. They like that a lot and they will remember you.
Something like, “I receive these calls frequently, as you can imagine ….I will always listen to your opportunity to see where I can be of help….I’m very happy here, working on ____ and ___…. As you explained the job specification, I’d suggest you talk to Jill Jones as she could be a candidate or know someone who would be. I’ve already done that job. My next move inside or outside of an organization will be (next level up) so that is where I’m headed….for now, I’m being rewarded for my contributions and very happy where I am….”
You always want to listen, be pleasant, be helpful. Then you remain in touch every six months of so with an update of new things you are doing and the offer to help by giving them other names in your network. (And that’s why you keep building your network or connections so that you do have people to suggest!)