Of course besides plugins, you can use visual selection mode (:help visual) which is esp useful with the { and } keys to move paragraph-wise. You can either use I to insert the hashes at the front or something like :'<,'>s/^/# (the '<,'> is automatically added as soon as you type the double colon). Visual block selection mode (:help block) makes getting rid of the hashes a breeze - move cursor to top hash, Ctrl-v, } to jump to the end of the paragraph, adjust width of block if necessary, d.

Makeshifts last the longest.