I found this on one of Palm's forums -
You can't hang up on individual callers once the conference has been initiated. You either hang up on both callers or ask one of the caller to hang up.
Hope this answers your question - On some of the newer devices with GSM carriers the software is different and will let you just hang up one caller.
OKC Sprint Tech
Unfortunately one of the callers will have to hang up first. Info below
Make a first call, then make a second call.Tap to end all calls.
Ask one of the callers to hang up, so you can stay connected to the other caller
When you’re done, do one of the following:
Tap to create the conference call.