There is no way to delete the history.
You can delete the times and start a new session, but as soon as you edit the history the times and session times re-appear.

The sessions are useless if after starting a new session all of the old session times re-appear if you edit a new history item.

Currently I work around this by keeping a template and starting a new tracker weekly by copying the blank template with the categories.

Other than that and the fact that kontact integration is pretty broken this is the best time tracker I have found and I have tried many so keep up the good work and thank you for ktimetracker.

Reproducible: Always

Steps to Reproduce:
1. Create some task / subtask and track some time.
2. Start a new session
3. Start and stop a new timer and edit the history for that timer
All the old session times re-appear in session

Also 
1. Create some task / subtask and track some time.
2. Reset all times
3. Start and stop a new timer and edit the history for that timer
All the old times and totals are back
Actual Results:  
Sessions and times re-appear after deletion after editing history for new time

Expected Results:  
If session is cleared, editing the history for a new task should not bring back the old session times.

If times are cleared, editing history for a new event should not bring back the old time totals.

Simply allowing the user to clear all history would fix this issue.
Currently you have to goto the history and delete each event one at a time.