Disabling Autofill is the second checkbox in the General section on the first page you see when you open Preferences in LastPass. It appears to be specific to each instance, so if you have LastPass installed in several browsers or on multiple systems, you may need to change it in each place.

You can also likely mitigate some of this by setting a fairly low autologoff timeout, though how well that will work may vary widely depending on how different people use the Web.