How can I block certain email addresses from being imported from a spreadsheet

A post-import cleanup would be more effective, resourcing to a batch campaign looking for email contains @somedomain.com and then deleting leads.

Please keep in mind imported leads remain "Marketo only" until an explicit flow step "Sync Lead to SFDC" is executed or a Program/Event is synchronised to a Salesforce campaign. 

If you have emails belonging to your competitors, why not take advantage of that? "Keep your friends close and your enemies closer" :)

Get them into Marketo, mark them as a competitor in some way, and keep track of them whenever they spy on your website. You could do this by specifically sending them a form in an attempt to cookie them.

From there you can decide what action to take: Simply keep an eye on their spying activity? Or purposely send them misleading information? The choice is yours :)

We use Marketo's black list feature to block all email addresses from competitor domains that make it into Marketo, and then we always make sure to include a smart list filter that says "Black Listed = False" on smart campaigns that send emails or sync to SFDC.

Here's a good method for identifying competitors in your database, and blocking them from future marketing: http://community.marketo.com/MarketoResource?id=kA650000000GtnmCAC

In addition to what Kate does, we also create a competitors list.  When we do our imports from Marketo to SFDC and do not want competitors in the mix, we will either use choice flows that indicate, if a lead belongs to the competitors list, remove from flow OR we filter our competitors from the smart list so they never run through the smart campaign.

Adam's suggestion is clever: tracking and scoring your competitors' activities gives you a leading edge.