But however, is there any rule that says having multiple account prohibited in this forum?
This isn't a question about the right to have multiple accounts or not. Owning alt accounts isn't against any forum rules.
This case is about a person who had one of their forum accounts banned. A ban doesn't apply to just one account, it applies to the person operating it. That means if one of your accounts gets banned, then all of them are banned because you (the person using the accounts) are banned.
Yes, that's the correct explanation of a ban which isn't even a matter of multiple accounts. There are users on the forum having multiple account and none of them is having any issue with multiple accounts. For example of the most respectable members LoyceV is having an alt account with the name LoyceMobile, and he's using the accounts without any issue because he isn't doing anything that's against the forum rules.
There is no rule which prohibits using of alternative accounts and a user can create and use as many alt-accounts as he/she possibly can. However, if any of the accounts is banned due to rule breaking or any other activity which is strongly prohibited and disliked in this forum, then all of the accounts should stop posting on the forum unless the users creates a ban appeal and the administrators accept that appeal. If a user is still operating any of those accounts even if his/her one account is banned then the person is basically ban evading, and that's not a good act at all.
In this case the HedgeFX's alt-account P2Pfinder was banned from forum, however despite the ban the user was still using the other account without creating a ban appeal for his banned account. In this case we can also conclude that the user was somehow evading the ban. However, I have seen this user posting many good and helpful posts on the forum, and he was using the forum as a good member. He was very active in lending board and he has taken many loans from lenders and repaid with interest on time.
That's why I think a user like HedgeFX deserves a chance, like @1miausaod there are many users which have been reported for plagiarism and some of them are still using the forum without any issues. I think anyone who has been using the forum in a good manner and is helping members of the forum with his/her knowledge should be given a chance after a ban. However, the final decision is always in the hands of the administrators, and if they give this user a chance then he will be able to use the forum once again without any issue, but if they condemned the user then that's also their decision.