SYNANON REJECTED ON ITS TAX APPEAL

A Federal district judge today dismissed the Synanon organization's claim to tax-exempt status as a drug rehabilitation program and religious group, ruling that it had engaged in systematic fraud against the court.

In dismissing Synanon's appeal to regain the tax benefit, Judge Charles R. Richey said the California- based group had engaged in ''willful, systematic and extensive destruction and alteration of documents and tapes'' needed to assess its right to the exemption.

The decision today was based on a case brought by Synanon in 1982 to regain tax-exempt status for all years after 1976. The judge, however, declined to rule beyond 1978, saying that for the more recent years, the group had not yet exhausted administrative remedies through the Internal Revenue Service.

Judge Richey said his dismissal was based on the group's ''egregious misconduct,'' amounting to a ''scheme to interfere with the judicial machinery.'' 'Chilling Portrait' Created

The judge also found that the group's own records and transcripts of statements by its leaders ''raise serious questions concerning Synanon's financial operations and create a chilling portrait of an organization that advocates terror and violence.''

Judge Richey said that Synanon's founder, Charles Dederich, had warned in a 1977 speech: ''Don't mess with us. You can get killed dead. Physically dead.''

Mr. Dederich and two other members were later convicted of conspiracy to murder a lawyer who had filed suit against the group on behalf of two former members. The lawyer was bitten by a rattlesnake when he reached into his mailbox.

Judge Richey dismissed the case ''with prejudice,'' meaning that the group cannot refile it, said that Synanon would have to pay the Government's court costs, and retained the right ''to consider the question of further sanctions.''

Neither Mr. Dederich nor Geoffrey P. Gitner, the group's lawyer in Washington, nor Philip C. Bourdette, its general counsel and a director, could be reached for comment. Judge Richey's decision noted that in an earlier case involving Synanon, the judge found ''Mr. Bourdette both committed and suborned perjury,'' Founding in California

Synanon was founded in California in 1958 ''allegedly to rehabilitate drug addicts,'' Judge Richey said, and gained tax-exempt status in 1960 because it was ''organized and operated exclusively for charitable purposes.''

In 1967 it began accepting nonaddicts in its residential facilities, and in 1975 it started calling itself a religion. It also began to engage in a variety of businesses, one reason cited by the Government for revoking the group's tax-exempt status. The Government also contended the exemption was not deserved because its earnings were diverted to private use and because its corporate policy of ''terror and violence'' did not conform to the requirement that a tax-exempt organization ''must serve a public benefit.''

In discussing the group's destruction of tapes and alterations of records, the judge said it began in 1979, contemporaneous with a revenue service audit aimed at assessing its right to tax-exempt status. A judge in an earlier case, he said, had determined that the group had ''deliberately destroyed'' evidence, and declared that ''Synanon has continued its misconduct and perpetuated this fraud up to the present.''