You can use Windows PowerShell to turn external sharing on or off. Using the SharePoint Online Management Shell, administrators can now enforce new controls over how external users accept invitations. When enabled, the RequireAcceptingAccountMatchInvitedAccount parameter requires external users to accept invitations with the email account with which they originally received the invitation.

• If this parameter is not set or is set to null:

When a user shares with an external user, they enter an e-mail like stephen@contoso.com, and an email is sent to Stephen at stephen@contoso.com. When he attempts to accept the invitation (by clicking the link in the email), he can log in with any account he wants to use. For example, he could use stephen@contoso.com, stephen@live.com, or even dwight@contoso.com. The sharing email can be forwarded and accepted by anyone. This system ensures that external users who use email aliases or who do not have a Microsoft account or organization account are able to accept the invitation.

• If this parameter is set to true:

The RequireAcceptingAccountMatchInvitedAccount parameter ensures that the user who receives the invitation is also the user who accepts it. If an invitation is sent to stephen@contoso.com, only a user who can log into stephen@contoso.com is able to accept the invitation. Any other email account displays an error page that directs to user to use the appropriate account.

https://support.office.com/en-us/article/Manage-external-sharing-for-your-SharePoint-Online-environment-c8a462eb-0723-4b0b-8d0a-70feafe4be85

https://technet.microsoft.com/library/fp161390

Using the SharePoint Online Management Shell, IT administrators can now require that all external sharing invitations be accepted only by the e-mail address that was originally shared with.

To enable this feature, simply run the Set-SPOTenant cmdlet and set the “RequireAcceptingAccountMatchInvitedAccount” parameter to true.

The parameter accepts two values: True or False.

True-User must accept this invitation with bob@contoso.com.

False– When a document is shared with an external user, bob@contoso.com, it can be accepted by any user with access to the invitation link in the original e-mail.

Administrators who desire increased control over external collaborators should consider enabling this feature.

You have first to download the online management shell

Then connect use the following command

Connect-sposervice

Then add the URL of the office 365 administration website

Which will be

EX: https://Domain-admin.sharepoint.com

Then you will be asked for the SharePoint credential to connect

username@domain.com

Then run the following command

Set-SPOTenant -RequireAcceptingAccountMatchInvitedAccount $true

Set-SPOTenant -RequireAcceptingAccountMatchInvitedAccount $Null

Set-spotenant

https://technet.microsoft.com/library/fp161390