Redmine

I would create a custom theme for this. Take a look at HowTo_create_a_custom_Redmine_theme on how to do it. It basically imports everything from the original theme and you can add additional css rules to it afterwards.

My additional rules would be...

#quick-search label, #q {
   display: none;
}

You could place the line at the bottom of the original stylesheet but then the changes are wiped out on every update of redmine.

Thanks for your help
I will try your solution