First solution:
here
Second solution:
I don't have Suitcase installed, so I'm guessing on this. But it's possible the login item is owned by root and not editable by whatever user you're logged in as.
See if there's an item in the standard applications startup items folder, which is here:
/Library/StartupItems/
This isn't the Library that's in your home directory, although you can check there too. To go to the directory above, open a Finder window, and press Command-Shift-G. You'll see a dialog. Paste the directory above into the text field and hit Enter.
If you see the Extensis item as a folder in this directory, you should be able to move it from there to your desktop, which should in effect render it unusable as a startup item. If it's owned by root, Finder will ask you for your username and password before you can move it.
If you don't see the Suitcase folder in that directory, also try looking in the launchd directory, which is another place that a file might exist which would start a service on boot. To do this, press Command-Shift-G again and enter the directory
/Library/LaunchDaemons/
You may see a file that contains "extensis" or "suitcase", or both. If you see one of these, try moving it to your desktop.
If you can do either of these steps, try rebooting and see what happens.