Deleting a user account from a OSX machine
It is safe to just use the System Preferences -> Accounts dialog to delete the user account. You always have to have one active account on the machine. You will need administrator access to the machine to delete the account. When you delete an account you'll be asked if you want to save the account's home directory or just delete them. Up to you which way you go there.

Now, if the user had administrator access to the machine, deleting their account may not remove all traces of the user from the machine. If they installed software in to /Applications or made system-level changes via /Library or any other means, those changes are going to persist on the machine. But if they weren't a privileged user, everything they could have changed on the machine was in their home directory and deleting their account got rid of it.

If you want to get rid of any trace a privileged user may have left on a machine you can delete the user account and then roll back to a point in time using your TimeMachine drive when that user didn't exist. Or, before deleting the user account, you can find all files that are owned by that user and manually deal with the ones that fall outside the user's home directory, then delete the user account. Or you can reinstall the entire OS -- but frankly that seems like a sledge hammer for a nail type of solution to me.