Make sure you also click: 

"Apply above settings to active and future listings"

Near the bottom of the buyer requirements.

If this buyer tries to buy again and has a problem because of the blocks, they may contact you to find out why they can't purchase.

You could just politely tell them that because they are a new buyer, your account is set up that they can't buy more than 5 items until they pay and get much higher positive feedback in their profile (or something along those lines).

If you ever need to see who has been prevented from buying from you - just have a look in the Buyer Log (same place you set up the requirements). It can make for some scary reading sometimes, LOL.