Thank you for your replay. We had the conversation with the support team, and seems this is a hard-coded requirement to issue an ACK from the ACK Translator. There are a number of use cases when a custom script is necessary and having an ability to suppress a response would be great.
One of the use cases is Quest Diagnostics to Epic results interface. Since Epic has developed a custom interface for Quest, including a custom ACK, such interface would be a complete pass-through for messages and ACKs. Certainly the logic for the custom ACK can be implemented in Iguana, but this require constant maintenance if Epic or Quest update their logic.