By adding lines such as:
blacklist_from sde@spledee.com
blacklist_from *@bonanzaoffers.com
blacklist_from *@deal-seeker.com
blacklist_from *@hispeedmediaoffers.com
blacklist_from *@jumpjive.com
blacklist_from *@*.ew01.com
to /etc/mail/spamassassin/local.cf or ~/.spamassassin/user_prefs , you tell spamassassin that mail from any of these domains gets a +100 spam score, effectively blocking them.
Then try whitelisting a specific domain? 
Otherwise if you are concerned about spam why not check out this black list:
http://www.sa-blacklist.stearns.org/sa-blacklist/
MORE:
The following procmail script will delete mail with a score of 15 or higher. By moving the # (comment) mark up one line, it will save all mail with a score of 15 or higher in a separate folder rather than deleting. The general implementation then, is that mail with a score of less than 5 goes into the inbox, a client side rule triggering on "X-Spam-Status: YES" will cause mail scoring between 5 and 15 to go into a Junk Mail folder where it can regularly be checked, and mail of 15 or higher will be stored on the server or dropped on the floor.
Code:
Uncomment the following 3 lines and use tail -f procmail.log to debug
#LOGFILE=$HOME/procmail.log
#VERBOSE=yes
#LOGABSTRACT=all
# Send all mail through SpamAssassin
:0fw: spamassassin.lock
* < 256000
| spamassassin
# Mail that is very likely spam (>15) can be dropped on the floor.
# Move the # up one line to save it on the server instead.
# Note that dropping mail on the floor is a *bad*
# idea unless you really, really believe no false positives will
# have a score greater than 15.
:0
* ^X-Spam-Level: \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*
/dev/null
#almost-certainly-spam
http://wiki.apache.org/spamassassin/...lToForwardMail
Read that for more information :-)
I am not certain what will happen if you blacklist *@* and whitelist a certain *@domain.com but give it a go. In order to delete the emails you will use the above and provide all blacklisted emails with a score say above 15 or 100 and then use that procmail script.
Let me know how it goes,