To put the blocks for members that have 1 or more strike in the last 12 months  in place hover your mouse over account in your my ebay page and from the drop down click on Site Preferences then go down the list till you see Buyer requirements and to the right of that click on show then click on edit. From there you can put in place the blocks you want, to stop those you want to block from purchasing from you.

And if you are manually adding ID's to your list are clicking on confirm?

copy and paste the ID's to make sure sure they are accurate.