Yeah, I fear that ‘we can´t disclose reasons’ is a widespread absurdity. Of course I understand that policy when other parties are involved and they can´t tell me because of that, but when it´s just me, them and my account, well.
But we have bad cards against those things, companies write it in their terms, you have to accept the terms if you want to use them and have to live with them. If it´s a company that is more or less in a monopolist position and you fall into disgrace with them for whatever justified or unjustified reason, tough luck.
Fraud is probably not the right word though, as you may have done whatever, even if unintentionally, to get them to block their account, or someone who you sent money or got money from did and they blocked your account because of something they only suspect but not know, can be anything of course, if they don´t tell you what. Unless you know, you are left guessing, and you´ll find complaints about all such companies though, if you look for it, never mind if Payoneer, PayPal, Skrill or what others there are.