i may be wrong, but most anti spam services work on email after it is downloaded & it simply moves what it sees as unwanted email to a folder besides your inbox. the nextel web client (i use it too) has filters but they are not really advanced to work as antispam service. so that being said, no, with the web client it is not possible to get only filtered email. if you are recieving an annoying amount of spam i would possibly suggest changing your email address. everyone views this as impossible but email everyone in your contact list, tell them you are changing your adderss after a set date, do it & possibly keep the old address for a while to monitor for any incoming email that you may want. its a thought.
best of luck