Symptoms

How does one disable or hide the "task actions" from the worklist control menu 

Diagnoses

This can be configured in the workflow. To “hide” the actions, run through the client event in question and uncheck the “Make the action available without opening the work item” checkbox on the “Configure Action” panel. (This option is checked by default.)

Resolution 

After un-checking the “Make the action available without opening the work item“ option, the actions were no longer available on the worklist control.

Additional Information…

Unchecking this option will remove the capability to action tasks through smartactions. 

Here are a few links for reference:
http://help.k2.com/onlinehelp/k2blackpearl/userguide/4.6.5/webframe.html_generic_wizards_-_configure_actions_.html
http://community.k2.com/t5/K2-blackpearl/Customizing-Worklist-Control-in-Smartforms/m-p/82596_M24685