You won’t get banned for joining bounties (no idea what you mean by “task”), but rather by what you do with your posts. The rules have been pointed out above, but just to emphasize on the main reasons get banned, I should point out:

-   Plagiarism: copy/[spin]/paste text from elsewhere, without referencing the source.
-   Spamming: posting constant low value content posts.
-   Ban evasion: persisting on posting from an account, when another account of yours (if you have one) has been previously  banned.

Besides the possibility of getting banned, there’s the possibility of receiving negative trust. This can lead your account to not being accepted in campaigns. The negative trust can happen for a wide range of reasons, many of which I personally do not agree with, but it’s not infrequent to see accounts tagged for being unethical (i.e. cheating in campaigns, using multiple accounts in the same campaign, meriting his own alt accounts, and so forth). The core reasons though should be commerce related.