Force.com IDE Developer Guide

Get Started with the Force.com IDE
Develop with the Force.com IDE
Test Code with the Force.com IDE
Deploy Code with the Force.com IDE
Get Started with the Apex Debugger
Get Started with the Apex Debugger
Set Up the Apex Debugger
Explore a Simple Debugging Puzzle
Manage Debugging Sessions
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
Get Started with the ISV Customer Debugger
Force.com IDE Release Notes
Useful References
PDF

Kill a Debugging Session in Setup

From the Apex Debugger page in Setup, you can easily kill any debugging session in your org, even sessions belonging to your coworkers. Please use this power compassionately!

User Permissions Needed
To manage Apex Debugger sessions from Setup: Debug Apex AND auto-enabled dependent permissions

OR

Modify All Data AND View and Debug Managed Apex
  1. From Setup, enter Apex Debugger in the Quick Find box, then click Apex Debugger.
  2. Click Kill Debugger.