Font Book won't disable certain duplicate fonts

I'm having difficulties editing events in iCal, and it was suggested that I go into Font Book and disable duplicate fonts. I was able to disable most of the duplicates (there were many), but I cannot disable duplicate Monaco and Helvetica Neue. I've tried every command (i.e "Resolve Duplicate" and "Validate Font", and it still won't take. Until I resolve this issue, I don't think I'll be able to resolve my iCal issue.
Thanks.