You can use the performServerAction API call to kill a session. 

(1)  Obtain the session ID from the Studio >> Manager >> Sessions panel (e.g. 3300878)

(2)  Open the following API call in Studio:
       /services/webservices/system/admin/server/operations/performServerAction

(3)  Enter the input XML for the call. 
       Below is an example of the input XML for terminating session ID 3300878:

<server:performServerAction xmlns:server="http://www.compositesw.com/services/system/admin/server" xmlns:common="http://www.compositesw.com/services/system/util/common">
  <server:actionName>TerminateSessions</server:actionName>
  <server:attributes>
    <common:attribute>
      <common:name>sessionIds</common:name>
      <common:type>LONG_ARRAY</common:type>
      <common:value>?</common:value>
      <common:valueArray>
        <common:item>3300878</common:item>
      </common:valueArray>
    </common:attribute>
  </server:attributes>
</server:performServerAction>

(4)  Execute the performServerAction call.

(5)  Inspect the Studio >> Manager >> Sessions panel to verify that the session has been terminated. Alternatively, you can run a query on the SYS_SESSIONS table in Scratchpad to check the session status: 

select SESSION_TYPE, STATUS from /services/databases/system/SYS_SESSIONS where SESSION_ID = 3300878

The result of the query should be:  STATUS=TERMINATED

Below is an example of using performServerAction to kill a Studio session.
   
     

Is there an API call that I can use to kill a session?