mysqladmin
is not a SQL statement. It's a little helper utility program you'll find on your MySQL server... and "flush-hosts" is one of the things it can do. ("status" and "shutdown" are a couple of other things that come to mind).
You type that command from a shell prompt.
Alternately, from your query browser (such as phpMyAdmin), the SQL statement you're looking for is simply this:
FLUSH HOSTS;