No, there is no way to disable your email address currently.
All email that isn't currently recognized will go to the "other" folder, you will not be notified of those.
Your Other folder contains messages sent to mailing lists and broad distribution groups, along with messages from people who aren’t friends or friends of friends.
Based on your email preferences (http://facebook.com/settings/?tab=privacy -> How You Connect), the sender may not even be able to send a message to your account and may receive the following message,
Based on the email preferences of the person you're trying to email, this message could not be delivered.
You could push a step further and report messages as spam.
From my experience, the Messages platform has been pretty seamless when it comes to the lack of spam. Then again, I have not explicitly made my Facebook email address known publicly.
So, to repeat, if you have not made users aware of your Facebook email address, there is little to no chance of you receiving an email from them.