MvvmCross Page Navigation Issue

I'm developing the app using MvvmCross. I need to clear all the back stack page & show the first page.
for ex: MainModel page--> navigationService.Navigate-->
SecondViewModel button click--->navigationService.Navigate
ThirdViewModel button click--->navigationService.close(this)

not clear the SecondViewModel. can you please anyone tell me how to clear & navigate to MainModel.