vacuum in single-user mode

A seldom-used database of mine was not recently vacuumed and I've run 
into the error: FATAL: database is not accepting commands to avoid 
wraparound data loss in database "stop"    HINT: Stop the postmaster and 
use a standalone backend to vacuum database "stop".  In fact, I get this 
error while trying to run "vacuumdb -U postgres -a" (and logged in as 
postgres).

I haven't the foggiest idea what this means but googled the error and 
found a site that seems to suggest that I need to run the vacuum in 
"single-user" mode before running VACUUM FULL, but I cannot find out how 
to do that, either.

Can anyone lend some advice, please?

Mark