in Lotus Domino 7.x (not sure if it is changed in R8.x) you can only add a *Possible_SPAM* to the subject (for example if you use Symantec Antispam or SpamAssasin) so that a rule can move this eMail to your spamfolder. But sometimes it happen that it isn´t spam. So the user send this eMail to another person... and ... you guess it... it will catched again as spam by the rule.
The best option to work with is, to allow us to work with: X-Spam-Level.
If X-Spam-Level is bigger that 7 -> delete
if X-Spam-Level is smaller that 7 but bigger than 4 -> move to spam folder
such kind of rules would be very very helpfull... (By the way... every other product can use X-Spam-Level.)