These are a number of enhancements for when leads are converted to Accounts and are left in the leads list. To gain the following feature enhancements please update to the September 2017 release.
This new feature set shows the Lead shaded with text that reads “Lead has been converted” in the Create New Lead Wizard results and in the Quick Search results. All previous converted leads will also take advantage of this feature.
Converted Leads will no longer have the “Convert to Prospective Account” button available if the Lead display form is opened.
To manage converted leads in the Leads list apply a filter that will not show them by default.
Note: This feature will default so there is nothing to do except update to the September release. If, however, at any time you wish to turn this functionality off, add the following Setting with a value of False.
Setting Name: ShadeConvertedLeads
Setting Value: False
**Further to the above converted Leads will also be shaded in the Outlook Plugin search results and also in the Contact drop down.