Right-click the message header and choose either to block the sender or to
block the sender's entire domain. Think about it first, because blocking the
domain might block people you want to hear from. For example, you probably
don't want to block the domain "yahoo.com", because you very likely know
people you like who use that domain.
It's my impression that most people living in places like Eastern Europe or
Asia use a very few major ISPs, which means that blocking domains may be
fairly effective. If you look in the Tools menu, Junk E-Mail Options,
there's lists of blocked and safe senders. I just don't think you can block
an entire country, e.g. block everything from *.cz or *.ru. Then again,
maybe those would work in the blocked sender's list. I just don't have any
easy way to test.
R.