Your database query should disable emailing of watched lists, I'm not sure why users would receive mail for watched topics if that setting was disabled. You should note, however, that we cannot support anything having to do with queries being executed directly on the database outside of the stock queries within UBB.threads ("Email Updates To Follow Lists" is a setting in the member management system as of v7.6).
If you really want to stop all emails coming from UBB.threads you could append "-- Invalid --" in the SMTP configuration (CP -> Master Settings -> Primary Settings -> General Tab -> Advanced Settings -> Use specified SMTP Server); which would stop all forum mailings, as you requested. This will also disable new registration, new private message, birthday, etc. emails.
Personally, I'd research all of the instances of "$mailer = new mailer();" and edit out the mailer coding for any emails you'd like disabled.
Please note that both of these are completely unsupported, especially editing of UBB.threads stock code.
