SFDC + Marketo Dependence

In Salesforce click on the Leads tab , Under the Tools section on that page you can click on Mass delete Leads, this will bring you to a page where you can enter the critiera for your Import. In step 3 you could enter created date = whenever (today)
Then in the a second critieria box you could select Created By Alias = Alias for your Marketo sync user

So build your critieria to point towards the leads you imported then hit search at the bottom of the page
then you will be shown the potential leads in a list and from here you can select what you want to delete 

You can also delete these leads in SFDC, from Marketo.  

Your smart list would be the static list you uploaded, and the flow step will look like this:

Steven's method is faster and more accurate in this situation. You can click on the Static Imported List, Select All, then do Actions > Deleted Lead from SFDC.

Be sure the counts from the list match the total # affected BEFORE you press Run.