I know more than 70% or more they using Bitcointalk Account For Bounties and other earning purpose.
I don't know where you got the figure but it is true somewhat. The real percentage can be a little higher or a little lower than 70%. Anyway, it is a big odds.
My some friends told me that bitcointalk account ban if they doing task or joining on bounties? Is it true?
Bounties and campaigns are not moderated and the forum does not run it. You should know the fact that bitcointalk.org forum is one of the free-speech forum but about rules it is most strictest forum. If you break rules, you will be punished. For some rules, you will have a few temporary ban (up to 3) before a permanent ban will be applied. For some rules (as plagiarism or ban evasion), when you break it and is found, you will get a permanent ban immediately.
Joining bounties or campaigns are not bad, and the forum allows you to join them. Spend your time to read the
Welcome message. If you won't spend time to read it, the forum is not for you.
How to avoid this things. I always wanted to know.
Read rules (others gave you above) and the welcome message, then do right things here.