Despite eBay saying buyer requirements in site preferences apply globally to current and future listings, they don't. If you change them in site preferences, they do not apply to current listings, and generally, any options you selected (or are selected by default), in one listing (be it current or ended) will carry over to future listings. If you're using the "sell similar" option from a listing that has that requirement, it will be automatically applied despite any site preference settings. Creating a new listing from scratch should have the current site preferences applied, though. (This applies to the online Sell Your Item form, it may be a different issue if you are using Turbo Lister).
If you have current listings with this requirement, you will need to remove it manually from each one.
If your requirements still revert to having that block selected, then (after clearing cookies, cache etc), I can only presume it's a site glitch, for which the only advice I can offer is to contact eBay and report it, but site issues can take a notoriously long time to be addressed (if ever).
