You can use tags for the desired tests. For example:
Scenario: Set off time in free time slot
Given I click first free time slot
And I choose menu item "Off"
And I enter time that is in free interval
When I click button "Ok"
Then I see offtime time slot with title that matches with saved interval
@wip
Scenario: Set free time on off time
Given I click last off time slot
And I choose menu item "Set working"
And I enter time that is in last off interval
When I click button "Ok"
Then I see freetime time slot with title that matches with saved interval
Then when you run lettuce just use
./manage.py harvest -t wip
It will run only those scenarios that are marked with tag @wip, in my case wip is for Work in Progress
If it is without django and just lettuce then use
lettuce -t wip