We experience a lot of traffic and server load on a web server.
All I can find out is majestic12 accessing pages all the time.
I wonder how I can prevent majestic12 from indexing the site
Do they respect any robots.txt entry and how do I write such an enty?
According to Majestic 12's own page about robots.txt, they fully respect robots exclusion (3rd answer from the top). The robots.txt file is a plain text file in the root of your website, i.e. you place it at:
and have these lines in the file:
So if you want to block that bot, I see no problem -- unless you're getting hammered by one of the fake bots they mention.
For the op's follow up question:
That would be a false flag operation. Virus disguises itself as a legit bot/process to crawl ip's. The explanation is on the majestic12 faq way down.