Its not considered spam when its part of the strategy. If the play field allows, they why not? If the opponent backs away from pressure then zoning is a must. Refer to this thread if you disagree:
http://www.trmk.org/forums/showthread.php/20154-Some-things-to-think-about-to-improve-your-skills
AGAIN, the term is not spam its ZONE. At competitive level, if the other player is dumb enough to fall for zoning tactics, then there are some things to reconsider.
The word spam is used for non competitive players. The word Zone is used for competitive use. The two are different in definition.