I think you need to specif your other device(s) EBS property to Null and NoDevice to something to make them not apart of your AMI. Your code should looks something like this.
//Create Drive Mapping List
var blockDeviceMappingList = new List<BlockDeviceMapping>();
//Create Mappings
var blockDeviceMapping = new BlockDeviceMapping();
var blockDeviceMapping2 = new BlockDeviceMapping();
//Specif a mount point of the drive you want (root)
blockDeviceMapping.DeviceName = "/dev/sda1";
var ebsBlockDevice = new EbsBlockDevice();
//Set something other than null constructor or u get an error about EBS not set. Likely has to do with how they build the request to send to the server
ebsBlockDevice.VolumeType = VolumeType.Standard;
blockDeviceMapping.Ebs = ebsBlockDevice;
//Specif a mount point of the unwanted drive and set EBS to null and NoDevice
blockDeviceMapping2.DeviceName = "/dev/sdf";
blockDeviceMapping2.Ebs = null;
blockDeviceMapping2.NoDevice = string.Empty;
//Add the mappings to the list
blockDeviceMappingList.Add(blockDeviceMapping);
blockDeviceMappingList.Add(blockDeviceMapping2);
//Setup Request
createImageRequest.BlockDeviceMappings = blockDeviceMappingList;
=)