Arcai.com NetCut can be especially useful if you suspect someone is using your WiFi without permission. In just a few seconds, you can block the access of any device. You can even see an intuitive graph with the percentage of the connection that each user is accessing. Plus, you can rename any connection, so you can clearly see which is which (so you don't accidentally block the connection to the wrong device).
Requires Android 2.3.3 or higher