Need to disable trackbacks on ALL entries

Try:

UPDATE wp_posts SET ping_status="closed";

https://codex.www.ads-software.com/Combating_Comment_Spam/FAQ#I_have_disabled_trackbacks.2C_but_trackbacks_continue_to_be_posted

To save running sql code all the time, this plugin should still work for 1.5.2 (but no promises)
https://wiki.www.ads-software.com/Auto shutoff comments

or

https://codex.www.ads-software.com/Plugins/Auto_shutoff_comments

You shouldn’t need to run the SQL code all the time, just once. Then they’re off. ??

But if he makes another post .. ?

If he makes another post, the trackbacks will automatically be closed if he has “unchecked ‘Allow link notifications from other Weblogs (pingbacks and trackbacks.)‘ on the Options > Discussion panel”.

Which he did as stated in the first post. ??

Thanks.