Preventing "Connection" and "X-Backside-Transport" headers is not possible for DataPower Web Service Proxy (WSP) and Multi-Protocol Gateway (MPGW) responses.
If really needed, this seems to be a general "header eraser" pattern for DataPower:
- chain request through pass-thru XML FW
- send request as HTTP 1.0 to prevent "Transfer-Encoding" header (eg. "curl -0 ...")
- eliminate any other header under "Header Suppression Parameters" in "Headers" tab of XML FW (direction: Front)
Find the details, a sample and some words of caution in this developerWorks DataPower forum posting: