To remove all lines starting with hast sign (#) from a file, try this:
sed /^#/d filename
The above command will remove all line starting with # from its output. If you want to write the output to another file, use o/p redirection >
sed /^#/d filename > newfile
But, if you want to over write the file with new o/p, use -i option with sed.
sed -i /^#/d filename