You can't block unsolicited offers directly. You can set auto-accept or decline if you accept offers.

As you said, if they are prepared to pay full price, they would buy at BIN. They are almost certainly scammers.

Scammers often target new sellers with relatively high priced items, as the assumption is those sellers are less likely to know the many pitfalls of selling online.

As far as the US offerers goes, there are 2 areas to address. As well as not posting internationally I am set up to exclude all regions except Australia. Might be overkill, but I don't get OS buyers.