Spammers usually make use of free one time use or 'disposable' email addresses when registering for forums. Here's a nice list of some of them. You should consider adding them to the list located:

Admin CP -> Settings -> Options -> User Banning Options -> Banned Email Addresses

A spammer will be denied registration if they try to register with an email address that has a domain matching one in the list.

@0815.ru@0clickemail.com
@0wnd.net
@0wnd.org
@10minutemail.com
@20minutemail.com
@2prong.com
@3d-painting.com
@4warding.com
@4warding.net
@4warding.org
@9ox.net
@a-bc.net
@amilegit.com
@anonbox.net
@anonymbox.com
@antichef.com
@antichef.net
@antispam.de
@baxomale.ht.cx
@beefmilk.com
@binkmail.com
@bio-muesli.net
@bobmail.info
@bodhi.lawlita.com
@bofthew.com
@brefmail.com
@bsnow.net
@bugmenot.com
@bumpymail.com
@casualdx.com
@chogmail.com
@cool.fr.nf
@correo.blogos.net
@cosmorph.com
@courriel.fr.nf
@courrieltemporaire.com
@curryworld.de
@cust.in
@dacoolest.com
@dandikmail.com
@deadaddress.com
@despam.it
@devnullmail.com
@dfgh.net
@digitalsanctuary.com
@discardmail.com
@discardmail.de
@disposableaddress.com
@disposemail.com
@dispostable.com
@dm.w3internet.co.uk example.com
@dodgeit.com
@dodgit.com
@dodgit.org
@dontreg.com
@dontsendmespam.de
@dump-email.info
@dumpyemail.com
@e4ward.com
@email60.com
@emailias.com
@emailinfive.com
@emailmiser.com
@emailtemporario.com.br
@emailwarden.com
@ephemail.net
@explodemail.com
@fakeinbox.com
@fakeinformation.com
@fastacura.com
@filzmail.com
@fizmail.com
@frapmail.com
@garliclife.com
@get1mail.com
@getonemail.com
@getonemail.net
@girlsundertheinfluence.com
@gishpuppy.com
@great-host.in
@gsrv.co.uk
@guerillamail.biz
@guerillamail.com
@guerillamail.net
@guerillamail.org
@guerrillamail.com
@guerrillamailblock.com
@haltospam.com
@hotpop.com
@ieatspam.eu
@ieatspam.info
@ihateyoualot.info
@imails.info
@inboxclean.com
@inboxclean.org
@incognitomail.com
@incognitomail.net
@ipoo.org
@irish2me.com
@jetable.com
@jetable.fr.nf
@jetable.net
@jetable.org
@junk1e.com
@kaspop.com
@kulturbetrieb.info
@kurzepost.de
@lifebyfood.com
@link2mail.net
@litedrop.com
@lookugly.com
@lopl.co.cc
@lr78.com
@maboard.com
@mail.by
@mail.mezimages.net
@mail4trash.com
@mailbidon.com
@mailcatch.com
@maileater.com
@mailexpire.com
@mailin8r.com
@mailinator.com
@mailinator.net
@mailinator2.com
@mailincubator.com
@mailme.lv
@mailnator.com
@mailnull.com
@mailzilla.org
@mbx.cc
@mega.zik.dj
@meltmail.com
@mierdamail.com
@mintemail.com
@moncourrier.fr.nf
@monemail.fr.nf
@monmail.fr.nf
@mt2009.com
@mx0.wwwnew.eu
@mycleaninbox.net
@mytrashmail.com
@neverbox.com
@nobulk.com
@noclickemail.com
@nogmailspam.info
@nomail.xl.cx
@nomail2me.com
@no-spam.ws
@nospam.ze.tc
@nospam4.us
@nospamfor.us
@nowmymail.com
@objectmail.com
@obobbo.com
@onewaymail.com
@ordinaryamerican.net
@owlpic.com
@pookmail.com
@proxymail.eu
@punkass.com
@putthisinyourspamdatabase.com
@quickinbox.com
@rcpt.at
@recode.me
@recursor.net
@regbypass.comsafe-mail.net
@safetymail.info
@sandelf.de
@saynotospams.com
@selfdestructingmail.com
@sendspamhere.com
@shiftmail.com
@****mail.me
@skeefmail.com
@slopsbox.com
@smellfear.com
@snakemail.com
@sneakemail.com
@sofort-mail.de
@sogetthis.com
@soodonims.com
@spam.la
@spamavert.com
@spambob.net
@spambob.org
@spambog.com
@spambog.de
@spambog.ru
@spambox.info
@spambox.us
@spamcannon.com
@spamcannon.net
@spamcero.com
@spamcorptastic.com
@spamcowboy.com
@spamcowboy.net
@spamcowboy.org
@spamday.com
@spamex.com
@spamfree24.com
@spamfree24.de
@spamfree24.eu
@spamfree24.info
@spamfree24.net
@spamfree24.org
@spamgourmet.com
@spamgourmet.net
@spamgourmet.org
@spamherelots.com
@spamhereplease.com
@spamhole.com
@spamify.com
@spaminator.de
@spamkill.info
@spaml.com
@spaml.de
@spammotel.com
@spamobox.com
@spamspot.com
@spamthis.co.uk
@spamthisplease.com
@speed.1s.fr
@suremail.info
@tempalias.com
@tempemail.biz
@tempemail.com
@tempe-mail.com
@tempemail.net
@tempinbox.co.uk
@tempinbox.com
@tempomail.fr
@temporaryemail.net
@temporaryinbox.com
@thankyou2010.com
@thisisnotmyrealemail.com
@throwawayemailaddress.com
@tilien.com
@tmailinator.com
@tradermail.info
@trash2009.com
@trash-amil.com
@trashmail.at
@trash-mail.at
@trashmail.com
@trash-mail.com
@trash-mail.de
@trashmail.me
@trashmail.net
@trashymail.com
@trashymail.net
@tyldd.com
@uggsrock.com
@wegwerfmail.de
@wegwerfmail.net
@wegwerfmail.org
@wh4f.org
@whyspam.me
@willselfdestruct.com
@winemaven.info
@wronghead.com
@wuzupmail.net
@xoxy.net
@yogamaven.com
@yopmail.com
@yopmail.fr
@yopmail.net
@yuurok.com
@zippymail.info
@jnxjn.com
@trashmailer.com
@klzlk.com