How to deny specific subnet from BGP update you already received from the neighbour?

prefix-list is used to filter your ingress or egress updates.If an update  already received at my end, I can simply make an access list and call it in my prefix-list for inbound traffic.

Use prefix-list for this purpose.

use prefix list that specific neighbor to deny that subnet 

we should use route map to block the route inbound direction

The redistribution of iBGP routes into Interior Gateway Protocol (IGP)—Enhanced Interior Gateway Routing Protocol/Open Shortest Path First/Intermediate System-to-Intermediate System (EIGRP/OSPF/IS-IS)—can cause routing loops within the Autonomous System, which is not recommended.

update 'prefix-list' associated to that particular neighbor by denying that specific subnet, later clear the BGP session with that neighbor softly. 

Using a Prefix list or Distribute list to that specific Neighbour will deny that subnet

advertise Prefix list with routemap  permit or deny