There are problems when creating a Contact without an Account if you have a private sharing modal. Contacts created without an account in this modal are ONLY accessible to the contact owner or profiles with View All permissions.
If you want to prevent the creation of an account you will have to have a few things in place first:
- Create a field on account and lead, type checkbox, named Created By Lead Convert
- On lead, default the value of this checkbox to true
- On lead, field mapping, map the field to the field on the account
- Then make the validation rule on account:
CheckboxField = TRUE )
this will prevent the lead from being converted if the user chooses to create a new account during lead conversion