The agreement is if they INTRODUCE a buyer to the property. They can not enforce any commission charge if they did not facilitate the sale by introduction.
You fail to mention if you have taken any legal advice on this! What did your lawyer or abagado say at the time you sold your property.
My advice is never trust the heresay on forums and go to a legal person who practices spanish conveyancing law!
I would tell them that you are seeking legal advice and your lawyer will reply to theirs!