While it is possible for regular members to spam, it doesn't happen as often as newcomers who come and spam the forums.
I believe after you've contributed your wisdom and your words to an area, you really get a sense of care for the community, as much of us have now. I don't think many regulars, would just forget the care they have for a community, and seal their fate by spamming and creating trouble..
Plus, not to mention I like to think we're all mature enough to know better anyhow.
I do think at least 90% of spammers on the forums are newcomers, and while there are different types of spam going on here, it does happen and when I see it happen (I report alot of it), it is 9/10 times a newcomer rather than a regular.