I don't think it's possible to prevent these kind of check with your method.
If the method is automathic, this mean that your script is easy checkable, if they do it manually, then:
1) There is no way to know what IP Plugrush staff (or every other ADS) use for check site and
2) For sure they don't use "www.plugrush.com" as referreal when they navigate your site
You can block the visit that they come with plugrush widget (when you buy or trade traffic), that's sure, but you can't block the manually checks.
I'm glad that others customers didn't got any problem, but i got hitted by a ban after i bought traffic from plugrush, that's a fact, not a random theory. So this mean that until you're under the radar it's fine, but if you do something that will force someone to check your site, you risk a lot.
And that's normal, i know, since it's a Black Hat tool. I'm not blaming you or your tool. I already said that your tool do the works, and do it good.
I'm only saying my experience (with a bad English, i know, it's not my native language).