The form has official ads, and the social norms here seem to tolerate advertising in signatures— or limited referral links in the context of places where you would naturally provide the same link absent the referral incentive. Creating a bunch of new threads purely to direct people to referral links clearly harms the signal to noise ratio here.
Vendors: Please don't create referral programs which are highly vulnerable to abuse, and to the extent that your programs can be abused by spamming please police yourselves by de-crediting users who promote through outright spamming. If you don't do this you may find your services completely blacklisted from places where they've become a nuisance. You can't claim zero responsibility for the foreseeable abusive behavior your programs encourage.