To accomplish only one record in the for the table is little bit tricky. but we can do it by over ridding the create method. here is my sudo code.

def create(self, cr, uid, vals, context=None):
    records = self.search(cr, uid, [])
    for id in records:
        self.unlink(cr, uid, id, context)
    return super("CLASS_NAME", self).create(cr, uid, vals, context=context)

Hope it will help.