Hi,
Are you using User Groups in your form, If you are using are you creating User Groups in the InfoPath Form itself or other way.
If you are using User Groups in InfoPath form itself, you can set a onload condition in code.
Just Verify whether the current user is in the Group or not onload, If he is in the group set a field say field1 (Put a field in Schema but not in UI) value to 1 other wise 0.
Then Conditional formatting on your item/section such that to disable the section if the field value is not equal to 1.