To turn the spam filter on or off go to the VPOP3 settings, then Settings → Spamfilter → General
Check the Enable Spam/Content filter on SMTP messages box to have VPOP3 process incoming SMTP and locally sent messages
Check the Enable Spam/Content filter on POP3 messages box to have VPOP3 process incoming POP3 messages
(Uncheck the boxes to turn the spam filter off)
Note that even if you get your incoming messages using POP3, it is a good idea to have VPOP3 process SMTP messages as it will detect locally sent messages and train the Bayesian portion of the spam filter what good messages are, as well as automatically white listing any email addresses that you send messages to.