Simply don't join "0".

For convenience you can always use a more comprehensive alias that will ensure all channels given to /join have a channel-specifier:

Code:
alias join {
  join $regsubex($1-, /([^\x2c]+)/g, $iif($left(\1, 1) isin $chantypes, \1, $chr(35) $+ \1))
}

Spelling mistakes, grammatical errors, and stupid comments are intentional.