How to clear the Time based workflow action queue in salesforce?

All pending actions to be triggered on a future date appear in the Workflow Queue. Here's how System administrators can view and manage the queue: Go to Setup | Customize | Monitor | Time-Based Workflow and delete the action which you don't want to fire.

To view pending actions:

1. From Setup, click Monitoring | Time-Based Workflow.
2. Click Search to view all pending actions for any active workflow rules, or set the filter criteria and click Search to view only the pending actions that match the criteria. The filter options are:

Workflow Rule Name: The name of the workflow rule.
Object: The object that triggered the workflow rule. Enter the object name in the singular form.
Scheduled Date: The date the pending actions are scheduled to occur.
Create Date: The date the record that triggered the workflow was created.
Created By: The user who created the record that triggered the workflow rule.
Record Name: The name of the record that triggered the workflow rule.

To cancel pending actions:
Select the box next to the pending actions you want to cancel.
Click Delete.