I think you guys are in the wrong direction. I downloaded that plugin and couldn't find the "You are detected as a spammer and Not allowed to register here" string anywhere so I googled it and found it here: https://github.com/amiyasahu/q2a-registration-blocker/blob/master/qa-registration-blocker-lang-default.php . So I'm pretty sure you have installed that plugin too.
So, as you can see the issue is on the q2a-registration-blocker plugin. I took a very quick look at the code and noticed this plugin checks on registration if the username, IP address or email are marked as SPAM in StopForumSpam. The plugin only performs that check if the "Ban Spammer IP Addresses from Registration" is enabled. However, it seems there is a bug there, because the plugin doesn't check if the the plugin itself is enabled or not. This means that even if you disable it, it will still perform the check. The only way to avoid that check is unchecking the "Ban Spammer IP Addresses from Registration".
You can fix this issue locating this file qa-registration-blocker-overrides.php and modifying this line:
if(qa_opt(qas_ubl_opt::BAN_SPAM_IPS)){
So that it looks like this (it is actually a single line but it gets wrapped in the answer):
if(qa_opt(qas_ubl_opt::PLUGIN_ACTIVE) && qa_opt(qas_ubl_opt::BAN_SPAM_IPS)){
I haven't tested this myself but seems to be the way to go. Once you're done with it unchecking the plugin enabled setting will actually disable it at user registration.