This will remove their watch lists period, so email isn't a factor.
DELETE FROM ubbt_WATCH_LISTS
WHERE USER_ID IN
(SELECT USER_ID FROM ubbt_BANNED_USERS)
This will just stop emailing of watchlist stuff, but not remove the fact that they have watchlists
UPDATE ubbt_USER_PROFILE
SET USER_EMAIL_WATCHLISTS=0
WHERE USER_ID IN
(SELECT USER_ID FROM ubbt_BANNED_USERS)
Both require mysql 4.1 or greater, for the sub-select
