End Workflow

how do u want to do it ? 

there is a action Stop workflow.

else u need to design a workflow a to stop it.

I would suggest write a service broker where you pass the proc inst id and in the service object u will get procinst.stop ()  . call this method and create a smart object and call this smart object whereever you want.

https://help.k2.com/onlinehelp/k2blackpearl/DevRef/4.7/default.htm#Start_Instance_and_Get_Data_Fields.html%3FTocPath%3DRuntime%2520APIs%2520and%2520Services%7CWorkflow%7CWorkflow%2520Client%2520API%7CWorkflow%2520Client%2520API%2520Samples%7C_____2