Force.com IDE Developer Guide

Force.com IDE UI Overview
Getting Started with the Force.com IDE
Developing with the Force.com IDE
Testing Code with the Force.com IDE
Deploying Code with the Force.com IDE
Getting Started with the Apex Debugger
Summary of Getting Started with the Apex Debugger
Set Up the Apex Debugger
Explore a Simple Debugging Puzzle
Apex Debugger Limits and Considerations
Apex Debugger Troubleshooting
Relaunch Your Debug Configuration
Kill an Orphaned Session
Kill a Debugging Session in Setup
Kill a Debugging Session with the Tooling API
Change Your Session Timeout Preference
Report Drastic Issues to Customer Support
Useful References
PDF

Kill a Debugging Session with the Tooling API

If you’re too cool for declarative tools, fret not: You can kill a debugging session using the Tooling API, without going anywhere near the Setup tree.
  1. Open the Developer Console.
  2. Open the Query Editor.
  3. Select Use Tooling API.
  4. Run this SOQL query. 
    1SELECT Status FROM ApexDebuggerSession WHERE Status = 'Active'
  5. Change the value of Status from Active to Kill.
  6. Save the modified row.