How to prevent selecting the wrong file format?
[Old question, but may still be relevant]

A quick but hacky option would be to comment out the IIF line in $optionTypes in CRM_Financial_Form_Export::buildQuickForm()

A better option would be create a setting and use that to set $optionTypes which would be a straightforward enhancement. Since a site probably wants the same type for every batch, a setting under CiviContribute Component Settings seems reasonable.