If your list is formatted with no http:// at the beginning then this should work.
sed 's/\/.*//g' file_with_urls
This will give you a preview of the output. If you are happy with the results then redirect the output to another file.
sed 's/\/.*//g' file_with_urls > output_file
sed -i 's/\/.*//g' file_with_urls
The -i will overwrite the original file with the new changes