Covid 19 gave us a dangerous RAT Parallax , do we have a solution to block the connection if we are unsure which file is causing this issue? My few desktops are being compromised.

There is no way that normal L3/L4 firewall prevents malware from being installed on computers behind it. For that one would have to use proxy server (the non-transparrent one) which does full anti-malware (and antivirus) scan ... which is not really possible to do "on the fly" while file (mail, ...) is still being transferred ... Well, some DPI (deep packet inspection) sollutions are aiming at performing such tasks, but DPI systems have much higher price tags than any of MT devices.