By A Web Design
To uninstall the Boxplus plugin in Joomla 1.7, you have to first login through the administrative panel of joomla by simply entering the url_of_your_website/administrator in your browser.
Enter the username, password and leave the Language to Default, click on Login as shown in Diagram 1.

Diagram 1
After logging in to the backend, a control panel page opens as shown in Diagram 2.
The Control Panel provides access to many default Joomla! functions and features. From the Control Panel, you can create and manage articles and categories. Other features available in the Control Panel are the media, menu, language, and user managers.

Diagram 2
In the top menu, locate Extensions. Click Extensions. From the drop down menu that appears click on Extension Manager as shown in Diagram 3.

Diagram 3
An Extension Manager page opens. Click on Manage as shown in the Diagram 4

Diagram 4
The Extension Manager: Manage page opens as shown in Diagram 5.
Select the check-box adjacent to Content - Multimedia display - boxplus and then click the Uninstall icon at the top-right hand corner as shown in Diagram 5. This will successfully uninstall the Boxplus plugin.

Diagram 5
Uninstalling Plugin was successful message will be displayed as shown in Diagram 6.

Diagram 6
This indicates that the Boxplus Plugin has been successfully uninstalled.