Removing Catch-All in cPanel without losing the main address?

[Oliy]

One of my clients has recently been getting a lot of spam to random_address@domain.com.

In order to fix this I planned to turn off the catch-all account, call it bob@domain.com and also have forwarders set up as so:

sales@domain.com -> bob@domain.com
enquiries@domain.com -> bob@domain.com

I then set the "default address" (for unrouted mail) to ":blackhole:" to simply delete it if it is not forwarded.

However, it doesn't work as I intended; everything is deleted, even ones forwarded on.

How do I set this up in cPanel to have a select list of "allowed" email recipients (all going into the same inbox) and for everything else to be deleted?

Cheers
Oliver