The site, which launched in 1996, covered all aspects of flight simulation, although its main focus was on Microsoft's Flight Simulator.
The attack took down the site's two servers and the owners had not established an external backup system.
The site's founder, Tom Allensworth, said that the site would be down for the foreseeable future and was unsure if would ever go back up.
"The method of the hack makes recovery difficult, if not impossible, to recover from," Mr Allensworth said in a statement.
"AVSIM is totally offline at this time and we expect to be so for some time to come. We are not able to predict when we will be back online, if we can come back at all. "