Rejecting "poor candidates" sends a message.
Sometimes an ERP written broadcast reminder to employees is sent saying: "Please pay attention to the qualifications specified on our posted the job descriptions (JD). Sending in less than qualified candidates slows down the assessment process...Happy to answer any questions you may have on the ERP."
On those rare occasions where an employee continued to send in less than qualified candidates--I'd call them in and remind them that their effort, while appreciated, is becoming a negative reflection on their grasp of knowing the difference between a "qualified" and "unqualified" job candidate. They got the message and started to pay attention the JD.
Some time ago I worked as an internal recruiter in a company in Sydney where the referral program in the contact centre had been a disaster. The reward for referral was so high that it represented a significant portion of a months salary so employees were often more focused on referring people than hitting their KPI's for which they got far less incentive. There was no cost and significant gain to the person referring. Furthermore the checks and balances were not there so it had created dynasties of extended families with the perception that you had to be related to someone to get promoted. I had the referring person calling me to check on their referrals application (privacy anyone?) and were quite upset when I told them I couldn't discuss it with them. It took 6 months to get some level of professionalism into the place and I don't think I took one person from the referral program in the year I was there.
My thoughts on the original question is to look at the motivation of the individual doing the referring and the program itself - fix that and you might fix the problem.