An ASN would be used by an ISP - it's basically an aggregate of multiple IP networks that the ISP is responsible for.
PPRuNe admins cannot do anything about this. We do not have any IP blocklists at the PPRuNe site level. That's not to say that there are none at the IB level (there are likely to be some) or at Cloudflare (DDoS protection). Is the error message from PPRuNe or Cloudflare?
You should be able to find out what ASN your IP address belongs to, or If you tell me the IP address that you are having problems with via PM, I can identify it for you. You can then use the Contact Us link to query IB admins as to why your ISP / ASN is being blocked - although as magyar_flyer has noted, if you are using a VPN (particularly a TOR VPN), it's not likely to be unblocked.