Generally Leads are only ever converted once the sales person has a positive feeling that they can be sold to.
This can be a gut feeling or it can be linked to a stage such as a product demonstration on meeting, or it can be linked to a defined set of criteria, eg. Did the prospect agree to receiving a quotation, did the prospect agree that there is a possibility of doing business, or some other similar 'check mark'.
We do not recommend converting a lead until this kind of criteria is met.
In theory this may mean that you will end up with a lot of leads that are not converted, which is completely normal.
However, rather than deleting them from the Leads, the normal process is that a 'status' type of field is used to define these dead Leads and then they are simply filtered out from any 'Active' status Leads that you may be looking at.
So they are there, but not there! ie. they can be filtered out of view in the screen you are looking at, using the CRMs 'view' function, which you can set to show all leads say that have a Status field value NOT EQUAL to 'Dead'.
The idea is then that you can re-churn these dead leads at a later date by doing email campaigns are telesales campaigns to see whether they may be interested in new products or services.
See attached document that shows this in diagram format.
Please refer to page 25 for lead definitions, then pages 28 - 33, specifically the diagram on page 31, which refers to keeping an archive of dead leads.