How to completely remove spam bots from Telegram groups?
This is because the bot that has been added sent a ReplyKeyboard as its ReplyMarkup. This ReplyMarkup are the button(s) you see at the bottom of the chat. To remove this you might want to add a bot that can send AND remove the Keyboard.

I know that @combot can do this with it's command !nokeyboard. This will send a keyboard to everyone in the chat and instantly remove it.

Hopefully this will help.