Users that already exist in the system with a matching username or email address will be completely overwritten by the syncing process.

You will have to update your LDAP query to exclude those users specifically or exclude an entire group, using the (!(query))  syntax. 

You can find some examples of the search syntax here: https://help.cheqroom.com/frequently-asked-questions/what-should-my-ldap-server-return-in-order-for-user-sync-to-work