Like i stated in my comment Chrome is installed on a "per user" basis so extensions added to a guest installation of Chrome will have no effect on your own.
But if you really want to restrict a user to not be able to install extensions you can revoke the rights to write (and possibly read) in the following directory:
For the guest account that would be:
Go to the
Default-directory for the guest, Right-click
Extensions and choose
Properties. Click the
Security-tab and click Edit. Click on the Guest-account and check the
Deny after "Full-control" and click OK.
You might want to take ownership of this directory too because clever guests can change the rights back (because they still own the directory and if you take ownership they can't). Although clever guest might just install a new version of Chrome to a different directory again so it depends on how smart your guests are.