Prevent syncing of indexed items that are otherwise synced

Perhaps this thread is relevant

the “hidden preference” to “Disable Automatic Updating of Indexed Items” is available in Help > DEVONthink Pro Office Help > Documentation > Appendix > Hidden Preferences

Before using Hidden Preferences of any sort, it is a good idea to open a ticket directly with Support (here or here or support_at_devontechnologies_dot_com), and confirm that the preference change addresses your personal situation.