If you assign leads to sales reps based on lead score, you can try this method which I've used in the past:
Add a picklist value to the Lead Status Field called "Recycle to Marketing". Instruct your sales reps to change the status to this when they talk to a lead who isn't unqualified, but also isn't ready for future sales qualification or an opportunity.
On the Marketo end, build a trigger that watches for this value change, and then have it zero out the lead score, and change the owner of the record to a marketing user (we used the Marketo sync account user). Reassigning the owner then takes the lead out of the sales rep's lead view and sales insight dashboard. I don't recommend deleting the lead from the CRM (though its certanly possible to do so), because you'll lose any activity history logged by the sales rep in doing so.
Then, when the lead eventually reaches the assignment threshold again, adjust your assignment rules or build a new one that reassigns this lead back to the sales person and changes the lead status back to "Open".
Hope this helps,
Jeff