You cant. At first it's just important that you get all the slots filled. Once that has happened, then you can start to adjust it over time.
For example, if you have a sponsor making you an offer, but you have no empty slots, cancel the highest prestige, lowest wealth sponsor. That way, when you go back to the Negotiation screen, they'll be offering you their money for the spot you just made available. You dont have to cancel, you can just wait until the 10 race agreement has run out. Its up to you.
That way you can pick and choose where your sponsors go. If you have a slot available, you dont get a choice. You can just leave that sponsor at making you an offer, though, whilst you get another sponsor to fill that spot... but then you're wasting 3 races of income and you're eventually going to want 5 star sponsors everywhere anyway.