Try removing the "EEPROM" jumper. That, supposedly, will disable the CD-ROM interface chip entirely.
I haven't seen any info to indicate that this is true and my experience indicates otherwise. The EEPROM jumper's purpose is to allow the programming of the EEPROM. That is when the jumper is closed you can program the EEPROM. Also in this mode the system is unable to use the info in the EEPROM and goes by the jumper setup to determine ports, etc. Opening the jumper allows the EEPROM to be read.
However, I did discover that you can disable the IRQ (or change it) using the configcd.exe program. You need to select the Creative CD200 drive as your CD and then you can have the most option to change/disable IRQs. If you choose a different creative CD or another manufacturer's drive then you are not given an option regarding IRQs (Choosing Mitsumi gives you a smaller set of IRQs).
Another thing to watch out for is if you don't have Win3.x installed. The configcd.exe program will ask the location of Windows and will instruct you to press ESC if you don't have Windows installed. However, this will just exit yu out of the program. You need to press F3 to bypass the Windows Directory question and then the EEPROM will be programmed.
Now, all I need is replacement mounting posts for the SCD-15 card!