not sure how to delete them but there is a way to have them not show up.
While in contacts hit menu, then view. You should see options on the bottom and you should be able to uncheck the ones you do not want to see.
I will try that
Go to SETTINGS, then ACCOUNTS AND SYNC select facebook and uncheck sync contacts and then do the same for yahoo.
back up contacts w/ backup assistant that is under accounts and sync.
then go back to SETTINGS then APPLICATIONS then MANAGER APPLICATIONS then ALL select PEOPLE then clear data. this will erase your contacts so make sure you have it backed up.
then go to SETTINGS then ACCOUNTS and SYNC then backup assistant and sync now and this should put all your contacts back on your phone without being relinked to facebook or yahoo at least i remember it being something like this when i was trying to get rid of facebook being linked to my contacts on my Tbolt.