I had similar needs some time ago and importer may have changed but it wasn't possible.
What I ended up doing was adding an expiry time to the data imported like today + 24 hours then mapping that to the expiry field. The cron runs every day and if an entry isn't included it will simply expire meaning it won't be available.
Has worked flawlessly for a couple of years now.
The site imports data daily. The data is supplied via customers database. We use Importer from solspace to call on that data and if it finds a record that existed we use the new expiry date within the data.
So lets say we have 2 entries in EE like
-- Entry01, Some field, some other field, Expiry date (today+24 hours)
-- Entry02, Some field, some other field, Expiry date (today+24 hours)
When Importer runs the next day (the data has been updated at their end) the only entry that still exists is 'Entry01' - So within EE 'Entry01' will be updated inc. new expiry date but 'Entry02' wasn't updated in the import so it will naturally expire.
This is all based on a daily import, it could easily be weekly, monthly or whatever – the key is setting the expiry date when you import the data.
CRON is a server side behaviour that runs a particular command at any given interval. Importer gives you the code for the CRON to use, you then set the timing in the CRON and off it goes.