How to revoke token for an user - vb.net

Question

How I can do to revoke token for a user?
This is my authenticate code and I want to reset (revoke) access for one user. This is because I use the GMail API and I want allow my user to change their email if necessary.
Actually the tokens are save with login at moment where token is reach but I must change this value when user decide to change their email.
Dim datafolder As String = Server.MapPath("App_Data/GoogleService.api.auth.store")
Dim scopes As IList(Of String) = New List(Of String)()
Dim UserId As String = Context.User.Identity.Name.ToString
scopes.Add(DriveService.Scope.Drive)
scopes.Add(GmailService.Scope.MailGoogleCom)
Dim myclientsecret As New ClientSecrets() With { _
.ClientId = CLIENT_ID, _
.ClientSecret = CLIENT_SECRET _
}
Dim flow As GoogleAuthorizationCodeFlow
flow = New GoogleAuthorizationCodeFlow(New GoogleAuthorizationCodeFlow.Initializer() With { _
.DataStore = New FileDataStore(datafolder), _
.ClientSecrets = myclientsecret, _
.Scopes = scopes _
})
Dim uri As String = Request.Url.ToString()
Dim code = Request("code")
If code IsNot Nothing Then
Dim token = flow.ExchangeCodeForTokenAsync(UserId, code, uri.Substring(0, uri.IndexOf("?")), CancellationToken.None).Result
' Extract the right state.
Dim oauthState = AuthWebUtility.ExtracRedirectFromState(flow.DataStore, UserId, Request("state")).Result
Response.Redirect(oauthState)
Else
Dim result = New AuthorizationCodeWebApp(flow, uri, uri).AuthorizeAsync(UserId, CancellationToken.None).Result
If result.RedirectUri IsNot Nothing Then
' Redirect the user to the authorization server.
Response.Redirect(result.RedirectUri)
Else
' The data store contains the user credential, so the user has been already authenticated.
myDriveService = New DriveService(New BaseClientService.Initializer() With { _
.ApplicationName = "Liens Google SyGED", _
.HttpClientInitializer = result.Credential _
})
myGMailService = New GmailService(New BaseClientService.Initializer() With { _
.ApplicationName = "Liens Google SyGED", _
.HttpClientInitializer = result.Credential _
})
End If
End If

Answer 1

You can revoke token by making a call to https://accounts.google.com/o/oauth2/revoke?token={token}
Read https://developers.google.com/identity/protocols/OAuth2WebServer