If you have an Admin role or sufficient editing permissions on a Class blog, you can follow these instructions to detach or remove student blogs from your Class blog:

Login to Reflect (visit: https://reflect.ucl.ac.uk).
Select the Class blog that you want to remove student blogs from, from the My Sites drop down menu.
Click on My Class and select Student Blogs.
Student blogs can be removed individually by clicking on the Remove Blog from Class button in each rectangle representing a student blog, and clicking on OK in the 'Are you sure? Student Blog will be removed' pop-up window.
Student blogs can be removed collectively by clicking on the tick button in top left corner of each rectangle representing a student blog, and then scrolling back to the top of the screen and selecting Remove from class from the Choose action drop down menu.
Click on Apply.

Take care NOT to unintentionally but permanently delete student blogs entirely, when you only wish to remove or detach them from your Class blog!