This brings back bad memories
I don't recall ever being banned from a site, but I do remember a colleague of mine getting very close.
I was working as the head of the support & implementation team at a small company that did software development and consultancy. We had a client we'd recently migrated to a new mail system. They then reported a problem.
I was working on another project so I sent my colleague John* to fix it. (This was in the early 2000s before remote support was commonplace.) After some basic checks John was very confused by what he saw, called the office to confirm that the system had the wrong settings, and then fixed it.
A day later, the exact same issue. I considered sending someone else, but I figured I'd send John again as he now had some familiarity with the site. When he arrived they were unhappy, but he cracked on and found... the exact same issue. We spoke briefly on the phone, and confirmed through date/time stamps on the configuration that this had been changed, and that only certain admin accounts had access to change it. John fixed the issue, reported all of this to the client, then left.
The next day they had the exact same issue. The client was livid. They were also demanding we send someone "more competent" than John. I happened to have finished my other work so I went onsite to give a show of "seniority", in the hopes it would calm them.
Same issue - the configuration had been changed manually. Before leaving the site I asked if they had a CD burner. They did, so I created copies of the configuration and burnt them to two CDs - one for us to keep, one for the client. This was so that we could prove a change was definitely happening.
Whilst I'd been on site poor John had been back at the office in a rather awkward meeting with one of our Directors about his performance. This was John's first job, and he'd not long been with the company, so this was not a fun experience for him. He was very worried due to all of this, fearing he'd have a career outlived by most mayflies. I reassured him and made sure to speak to the director the next day, making it clear I trusted John completely and didn't trust the client.
Two days later, the client has yet another failure. I went out to the client, as they were still unhappy with John. It was the exact same configuration change. But this time I invited their IT manager to compare the backup that they had with the current configuration, and showed the date/time stamps on the current configuration. I made it clear that someone with admin access to their environment was making this change and sabotaging their system. With no other explanation available, he accepted that this. I fixed the problem, took another backup for evidence in the future, and went home.
A few days later our director spoke to me about this. After some internal investigation the client found that one of their technicians didn't like the new system, and had been sabotaging it in the hopes that they'd roll back to the old one.
I made sure that John got an apology from the director, but was less than happy that they'd not contacted myself or John to apologise.
When the same client had some deployment work a few weeks later, I allocated the visit to John. He didn't want to go, but I told him that I expected them to apologise to him in person, and to be more professional with him from now on. If that didn't happen, I'd never send him there again and would tell our management that this client wasn't worth the hassle and we should not renew their contract when it expired. To the client's credit, they apologised to him and treated him well.
Some clients simply aren't worth the hassle that they bring you. Being banned or unwelcome at a site is one of the (many) danger signs.
Looking back on it, I'm mostly happy with how it went. I should have asked John to take a backup after his second visit, but didn't know that they had a CD burner at that point. We did change our standard operating procedures after that, making sure everyone took spare CD-Rs onsite and offered to create a backup of configs before leaving.
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* Name changed, of course.