I don't know much about restraining orders. But doesn't there have to be an incident first in order to get one? It sounds like they're annoying and total wack-jobs but I'm thinking there either needs to be a threat or they have to have tried to do something to you before can get a restraining order. You might call the polics department's information number and ask. If you know a lawyer, you can probably get them to write a "cease and desist" letter to the wackos. Something along the lines of "you must cease all contact with my clients including phone calls, etc. or further legal action will be taken." This might be enough to get them to back-off.
Here's what I think I would do: It's time to totally ignore them. Don't take their calls. Perhaps there's some way DH can have their number blocked if they try to call him at work? Maybe his tech department can help with that? If he does pick up the phone and it's them, he should say he's going to hang up and then do it. Don't engage them in conversation at all. Don't listen to anything they have to say. They seem to enjoy getting a rise out of people. You and your DH need to take that power away from them. Don't look at them on the street. Don't slow down or look at their house when you drive past. Don't explain to them about your medications. It's none of their business. Basically, treat them like they don't exist. This may make them mad at first, but they'll eventually realize their tactics aren't going to work and move on to annoying someone else. If there's any more poop in the mailbox or any other sign that they've entered or vandalized your property (and you're sure it's them), document everything and report it to the police. Maybe a visit from the friendly neighborhood police officer would scare them into submission.
Good luck with this. I'm sure it's not easy to deal with people like this!