On the machine from which you are connecting with in order to attempt a connection kill, you need to enter the name of the SDE service along with a port number into your services file. The services file is located at the following locations depending on OS:
Windows 95 - C:windows
Windows 98 - C:\windows
Windows Me - C:\windows
Windows 2000 - C:windows\system32\drivers\etc
Windows XP - C:\windows\system32\drivers\etc
Windows NT - C:\winnt\system32\drivers\etc
Windows Vista - C:\windows\system32\drivers\etc
Toward the bottom, make an entry like the following:
esri_arcsde 5152
The esri_arcsde should be replaced with the actual name of your ArcSDE service and the port number should be the same as the port it's running on. Use a tab in between these values. Also, be sure the services file has a blank line at the very end of it so that the ArcSDE service is not the very last line in the file.