It is possible to delete multiple plugins at once but by default WordPress doesn’t provides you any feature for deleting multiple theme at once. Bloggers and webmasters, they add multiple themes for testing purposes but it becomes very time consuming job to delete themes one by one from admin area dashboard, the only way of deleting all themes at once is deleting all times directing from wp-content/themes directory via FTP or cPanel.
This tutorial on new WordPress plugin that adds a new feature that allows you to delete multiple themes at once, right from your admin area dashboard. Install and activate Delete Multiple Themes. After activation visit Dashboard -> Appearance -> Delete Themes admin page and here you can choose the themes you want to delete Also you can see which themes has child themes installed.
It is very important to remove all unnecessary themes plugins as they may cause security holes in your WordPress installation.