This is similar to removing a Lead. Except for the fact that you may want to report on the leads lost.
Hence you will only have to abandon the lead and not delete it.
Select Jobs > Detailed Leads
Find the lead that needs to be removed and click on the lead number.
The left navigation panel will expand with all contract (lead) details. The heading "Contract Details" will also gain focus.
Expand the Lead Number
Select "Management" under the lead
Choose Abandoned from the list and choose the appropriate reason.
You can have pre populated drop down. This list can be created if you have the admin rights.
else you can enter the reason under 'OtherReason:' section below.