Hi. I work in a sort of Hub, within a secondary school. We work with some challenging young people who are adept at avoidance! As such, they will come to us for a drink of water during the school day. However this is proving disruptive to the pupils who are scheduled to be with us at any one time. Also, we want to teach them responsibility; carrying a water bottle, getting a drink during break and lunch, etc. Basically, we need to clamp down a bit.
I thought it would be useful for them (and us!) to have a visual reminder on the door. Something along the lines of 'please respect other Hub users by asking for water and phone-charging during your timetabled Hub session only'. This is a bit long-winded, however.
Our approach is generally positive and nurturing, so it can't come across as too rude or abrupt! 
Ideas on a postcard well, a door sign please 