However some users did it while participating in bounty campaigns and ICOs wherein they followed rules that required them to do so and even though it is wrong, I feel such participants should be given some warning and signature bans instead of being permanently banned.
The "rules" of an ICO or bounty campaign do not trump the rules of the forum. If you plagiarize, you will be banned.
I think an exception can be made in a minority of cases where the user is obviously a good contributor to the forum, with a history of useful and constructive posts, an extensive positive trading history, huge numbers of good reports, etc.,
and the plagiarism wasn't made for purely selfish/financial reasons. In these cases, a temporary or signature ban is acceptable. The vast majority of cases do not fall in to this category however, and certainly not the example you have given. People plagiarizing for the sole reason of earning money through a signature or bounty campaign deserve a permanent ban.