No. Spammers have found this to be a wonderful boon. You might not want to follow a hashtag as you'll likely be flooded with spam. But since I'm more interested in empowering you, rather than telling you what you should or shouldn't do, I'd happily merge a PR that added this and I might write it myself eventually.
Thanks .. and the reason I ask this .. in an enterprise set up .. many conversations are driven on hashtags .. communities or rooms are just too cumbersome and formal .. Being able to follow a hashtag allows me to follow a cross conversation .. for example if I am in customer service .. people can respond to customer with #service .. then any agent can pick up the message and run with it .. as for spam .. that is a problem #nostr should anyway handle .. it should not limit/restrict the good usage ..