So as a joke we invented a fake employee with a fake name. I think his first name was Joe. We gave this person an email address and I made a voicemail recording for him.
When a time-waster was bugging us we'd tell them to contact Joe, "who is in charge of that" and provide the contact info. What started off as a joke ended up saving us a fuckton of time, effort, and hassle. For some reason Joe's incredibly upbeat and douchy sounding voicemail message was like manna from heaven for the relentless salespeople that refused to stop calling. It was an interesting social experiment of sorts. Joe was essentially a /dev/null bucket and never responded. But it worked.
This lasted for years and was really beneficial for us. Harder to pull off these days, but not impossible I imagine!