iMac tends to select non-prefered wifi

I have the same wifi accessible through two different routers. Although I have set my network preference to one, I find the iMac tends to revert to the other (not preferred). This is only a problem when I seek to print, as my printer is on the first network. I've had this problem with each Mac Os. How do I get the iMac to respect my preference? Is there a secret switch I need to flip? Do I have to restart the computer to clear some sort of hidden record of the other network?