I want to resell my space but
when one logis in to the webmail at the top right corner it says system hostname blabla.com
How can I get rid of that?
You can set that name in Webmin configuration, Webmin -> Usermin Configuration -> User Interface : Hostname to display in Usermin.
Hey That worked . Thanks a lot 
what if I want to remove a particular word
blabla.co from entire virtualmin system every where?
You’re welcome.
What do you mean exactly with “a particular word” and “the entire Virtualmin system”?