That does the paperwork say? Generally there are terms and conditions that must be met. If your paperwork says something to the effect of "property and remaining all items contained within" then the table belongs to the new owner at the time of the legal closing date. If there is no clause then technically you have to wait for the required waiting period for said property to be considered abandoned. Generally its 30 days but can vary.
You might want to read through this
http://www.qp.alberta.ca/documents/Acts/U01P5.pdf
Your best bet would be to send a written notification to the previous owner via registered mail requiring a delivery signature demanding that the pool table and all accessories be removed by a certain date or they will be considered to be abandoned and at that time become the property of the new owners. I would also be in contact with bylaw to inform them that you have reason to believe the PO is trying to extort your parents and request they get involved.
Lastly when your parents bought the house they must have had a lawyer involved so perhaps talk to them about it.