If it is not authorized by the owner of the structure or building it is made on, graffiti is punishable by law, just like vandalism or misdemeanours. There are several possible legal consequences if you get caught making graffiti:
+ Pamphlet for store owners - SPVM
+ Pamphlet for parents - SPVM
+ Les graffitis : une trace à la bonne place ? - Ville de Montréal (french only)