If your default setting is put manually to "By latest replies" (= 0) but some users still receive the other sorting, this can mean only that in their personal preferences it's set to "By latest topics".

Simply tell these users to switch their personal setting to the other thing :)

Ivan
nomadicink has united database, so Preferences are disabled.

nomadicink
There is nothing to do with cookies in that case. As I told you, you need not just to UPDATE all user_sorttopics field values to 0, but also set in table preferences (via PHPmyAdmin or whatever), so when NEW user registers, field is updated to 0 AUTOMATICALLY. This is what I meant under "default" value.