Limiting user access by subnet to Amazon Aurora PostgreSQL
here are two ideas that can be tried..

Run a script or other little app in the background that scans through the killing connections that do not match the established rules
it fires off every few seconds ..

Or create a system event trigger that fires off checks the user IP address and any other information that needs to be checked when the connection is made,  if it fails the checks  call

select  pg_terminate_backend(pid) --or
select  pg_cancel_backend(pid)


The information needed is located in 

select * from pg_stat_activity