If you enjoy conversing with someone, reply back with a "Hi, how's it going?". If you don't want to talk with them, you're doing the right thing by ignoring them.
As for ending a conversation, I like to end my conversations since I often don't realize someone is still chatting if I don't hear from them for about 5 min. Therefore, whether I'm ending abruptly or not, I'll end the conversation kind of like a phone conversation, by saying something like "thanks for the chat, talk to you later." If I have to leave abruptly, I'll say, "sorry... gotta go. ciao 4 now!" If you'll be returning to the chat after a phone call, of course, you can say that, like "I got a phone call. BRB"
Anyway, you should feel free to conduct yourself as you would through any medium of conversation. If it's just not your thing, ask the person to contact you by email or phone if that's what you prefer.