I doubt that you will find a nice lean set of rules for these forums, but there are rules that are part of the Rewards program.. What has been said, repeatedly, is that promo-code spamming will not be tolerated, on the internet as a whole, or on these forums or the DH Wiki.
Jeff, from the DreamHost abuse department, has
said most recently:
"It is absolutely unacceptable to:
5. Spamming forums - including our own - in order to lure prospective Dreamhost customers to your site in order to obtain Rewards using thinly veiled promotional techniques."
Over time, and much discussion (some of which has been very heated), it appears to have evolved that a promo code/link in a sig only, on a post that has a legitimate purpose other than to post the link, is acceptable.
Linkspamming in the forum as a promotional technique to drive traffic to your site has also resulted in post being deleted by Dreamhost. Note that some of the most prolonged flamefests on these forums have arisen from this whole topic (and going back many years)!