You can view the history of each time you have logged in from a particular device or through the web, as well as the time, general location and IP address. To view your login history, click account then login history.
On this page, you can choose to remove a device, which you can also do on the devices page.
If you choose to Remove a device, you can simply remove it and disable its access to the account. You can also choose to remove it and remove any files downloaded to this device by Syncplicity by using the Wipe and Remove option, if available.