Eircom have actually been told by the telecom regulator that they cannot use their lists for such winback calls as it is anti-competitive. I had the same situation when we moved our line to BT. In the end I asked to speak to a supervisor, and then to their manager. Got their name and an undertaking to take me off their list or I would make a complaint to ComReg. Never got called again.
The key is to be polite, but ask to be transferred to a manager and do not take no for an answer. Then take down their name 'in case there is a problem, then I will know to get back in touch'. Call centre staff have vitrually no authority to do anything, but managers can.
cheers,
Diziet