I have some suggestions for the complete version of the Ubuntu Forums Rules, the one you have to read on registration.
First, I would call it Rules & Guidelines instead of just Rules since much of it is just that, guidelines.
Then, I would clarify the term spam (cf. I.6 & III.2).
Finally, I would add the following article on advertising:
The Forums are not meant to be used to promote products, people, organisations, political groups religions or ideologies. Such matter is forbidden unless its clearly and meaningfully related to the theme of given forum. Linux related advertisements are allowed in the signature.