You manage to get the agent? Or go to HDB? I am sure your brother has the sales and purchase agreement, where the IC number of the seller is there. Sure can find them. Or go police for help, or go MP, get the address of the seller, tell the loan sharks that their guy is at the certain new address.
Second option is to install a CCTV, tell them that they are being recorded, and if they come again, the tapes will be handed to police.
Hope this helps