I had this issue of teachers borrowing and requesting stuff from the art
department a few years ago, and every-once-in-a-while it cropps up again
(usually because it is a new teacher to the school and they don't know). I
took the oportunity in a staff meeting to politely remind the staff that I
am not the warehouse of the school, and that the art budget is not intended
to cover the entire school. They seemed to understand this. I also
reminded them that they are welcome to see me about borrowing items (and as
long as I have notice and we're not using them it is usually not a problem).
Some things I really don't mind - ie: newsprint...I have tons of it. I have
found over the last few years (after the staff meeting reminder) that
departments have been buying their own department supplies so that less and
less people are bugging me to borrow the scissors, etc. Although it took a
while for these departments to do this, it is of course the best thing!
Also, if students come to my door while I am teaching I usually do not
accomodate them at all....as they are interupting my students.