I was left a comment (probably the first time in all the time of the YouTube channel) with obscene words.

I have a few questions:
Can the channel be blocked for obscene language?
How can I moderate comments?
What's your advice?

Use a blacklist of keywords. Make a list of words that should be blocked in your chats and add them to the Community Settings.

You can appoint moderators yourself, which will allow you to not be distracted from the broadcast to manage the chat. A moderator can reply to viewer messages, delete inappropriate comments, and create a list of inappropriate words (comments with them are blocked). You can see more detailed information here https://chekkee.com/how-to-moderate-your-youtube-live-stream-chat-comments/ . Good luck!