Implicit sharing is automatic. You can neither turn it off, nor turn it on. It is native to the Salesforce Platform. In other words, this isn’t a configurable setting available in Salesforce.

To remove User’s implicit access to the account, you have to remove his/her access to Opportunities, Cases, or Contacts under it. Develop a batch to take care of this.

Using Mass Transfer records will allow transferring Accounts, Opportunities, Contacts and Cases.

1. Go to Mass Transfer Records.

2. Click Transfer Accounts.

3. Select the appropriate options.