It is not possible to disable contact creation. However, you can setup autofill of landing page fields that will help to avoid he duplicate contact records creation
When the same case is created through email the following business process is triggered "Incoming email registration process" where only the case got created (No contact creation).
What runs to create a Case in Creatio CRM site via the Landing page? and What triggers to create a contact when a case is created?
This logic is hardcoded and cannot be disabled without changing the records creation from web forms. It doesn't have anything in common with cases creation from emails. The above article is the solution to any web forms that may create duplicate records.
