I don't know if there are any programs that can filter it for you, as they would most likely have to be server side, but I know that you can custom set filters:
To create a rule using the Lotus Notes R6 client program please do the following:
1. While in your inbox view click on the plus sign next to "Tools" and from the submenu select "Rules"
2. In the top menu bar click on the button for "New Rule"
3. Make sure that the option for "This rule is" set to the on position.
4. Create your condition that the rule will act upon using the pulldown menus. Once your conditions are set hit the "Add" button
5. Now you will need to create an action that the rule will carry out when triggered. Do this by using the pulldown menus in the "Specify Actions" area. Once your action is set hit the "Add" button.
6. Once your rule has been set hit the "OK" button to save your rule.
To block a particular e-mail address:
To block an account in Lotus Notes:
1. While in your inbox view left click once on the message from the person you wish to block
2. Once the message is highlighted click on the menu option for "Tools" and select "Block Mail from Sender"
3. In the new window that pops up you will have an option to create a simple rule to block the sender. Hit the "OK" button to save the rule.
4. Then from now on whenever you receive an email the address/person you specified that message will automatically be moved to your "Junk Mail" box.
Don't know how much this will help, as it's not automatic, but I know how much of a pain spam can be. My dad uses Lotus Notes for work, and get gets about 500 emails a day, and half of them are spam.
I don't know if you're version has support for blocking specific e-mails. It said somewhere that it is new to release 6, but at least give it a try.