I would create a smart campaign. Use created at date field to narrow down the search and then reference the field you have populated with the #REF! value.
Filter 1 = Lead Created Date = Day of upload
Filter 2 = Field with data = #REF!
This should create your smart list of 20k leads. I would then delete those leads from you system. See my screen shots below: Make sure you double check everything. Are you linked to Salesforce? Have the leads also been created there too? You will have to remove them from there as well or they will sync back across. This method will delete from SFDC but check with your SF admin first to confirm everything and always double and triple check this is set up correctly before activating (I remove myself from any responsibility for what you are about to do) .
In my screenshots I used the field Anchor Text, obviously you would swap this with whatever your field is.
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By the way have the new leads been created without data in other fields like name, company etc? If that is the case I would add extra filters to your smart list targeting those fields with value of "is empty". Again this is just another way to make sure your not deleting anyone by mistake and an extra step in minimizing risk.
Thanks a ton @Gerard Donnelly.