Shantha Kumar T
Follow the below steps to disable the sharing option for outside your organization,
Step 1: Go to SharePoint Admin center
Step 2: Select the site collection and select the Sharing from ribbon
Step 3: From the Sharing dialog box, select “Don’t allow sharing outside your organization”
Step 4: Click Save.
Step 5: Click OK in alert message.
In few seconds, the settings will updated to the Site Collection