If you've enabled the Reclaim's Slack integration, you delete a Task from anywhere you can find the task:
@Reclaim may have alerted you in the Messages before it was time to start the task.
The @Reclaim Home tab has an Agenda view. Pick the date of the Task Event to find the Task.
The @Reclaim Home tab has a Tasks view. There you can find the Task Event under Recent Tasks.
When you find the Task in Slack, click the Delete button.