Pipeliner CRM does this search too, automatically.
Implementation of this can't be an no, just do. =)
Great feedback! We recently added an option for Any/All selection with Tags, and we’re taking a look into adding an Exclude filter as well. More information to come when we have it!
Over all the filters, just add a slider (include, exclude) to choose if you want to include filters or exclude filters.
Or every filter get a checkbox to exclude that filter (as default it includes) checking the box excludes.