If groups have been set up for targeting purposes, then it is not possible to untick an individual within the group.

You can add individuals manually by selecting users, but you will forfeit the right of using automation for new joiners. 

For further information, see articles:

• What should I consider when switching to SCIM User Autoprovisioning?
• If I change to SCIM User Autoprovisioning, will I still be able to use the Collectives and User Groups I have created?
• Will new Joiners and Leavers in my Organisation be added to the platform and targeted with the relevant Content after switching to SCIM User Autoprovisioning?