Are you coming at this from a licensing standpoint instead of a security standpoint? After all, developer licenses are relatively expensive and most organizations have a limited number of them. If this is the case, you could remove the developer authorizations, but in order to get the license audit to stop counting the user as a developer for license purposes, you would need to remove the DEVACCESS entry (and you should also remove the developer authorizations). Additionally, you may need to go into the SSCR key administration tool on http://support.sap.com and remove the developer key there as well.
Or, if the user is gone, simply invalidate the account.