Removal of Security

You cannot delete a security, because that would mess with your past performance and perhaps with account balances.

The first thing to do is to determine what Quicken thinks you hold. If you open Window > Securities and find this security is there a green checkbox in the far-right Currently Held column? If so, your Quicken data has you holding this security. 

Next, go to Investments, or the Account this security is in, and look at the Portfolio tab, with your filters set to view by Portfolio Value and Group by Security. do you see this security in the list? What does it show for Shares held? It's not unusual, especially if you migrated to the current Quicken Mac from Quicken 2007 or Quicken Windows, for a security to show a tiny factional number of shares due to rounding and different ways of recording transactions in the past. If Quicken shows you have .000001 share of the security, create a Remove Shares transaction to remove that bogus residual amount, and the security should disappear from the portfolio view. 

Well, what happened to those shares of the security in real life? Assuming you sold them somewhere along the way, is that not reflected in your Quicken? One would think something else in your Quicken must be off, because the proceeds of your liquidation of that security should have affected something else in your Quicken accounts.