I have Farkas from The Companions following me. We were supposed to slay a dragon together, but I had already killed the dragon myself. I have no option when I talk to him to have him stop following me, I also unable to kill him or ask anyone else to follow me.
Just to add for future notice, this can happen to Vilkas as well, so if you're doing Companions related stuff, just tell them you're not interested or no if they bring up dragons when you prompt a conversation with them.
"It's a room of politely worded death, Senior!" "I got your money. I'm going to get a lap dance."
It'd be funny if you could take him to the nearest frostbite spider since he's afraid of them and should run off. But since that won't happen have you done Boethiah's calling? If you manage to kill him there I think companions respawn later (as your follower of course).