Hi Jayanth,
We recently asked Marketo how to best tackle this issue their suggestion was as follows "...you could create a campaign with a filter looking for "Anonymous" equal to "True" which would bring up all anonymous leads. You could potentially filter on things like created date if you want to take out older records. Then in the flow have it delete the leads..."
However, we use multiple workspaces so instead we decided to create an Unknown lead partition and made these only viewable by a "Unknown" workspace with triggers to change the lead partition if/when they become 'known' in the future.
I hope this helps.
Kind regards,
Victoria