LET'S HEAR IT -- a little toot, anyway -- for Sen. Jesse Helms, author of the U.S. Department of State Freedom of Expression Act of 1987. It's his way to punish the State Department for a small but resonant act of bureaucratic trampling on free speech.

For years the Washington police and the U. S. Secret Service, to protect visiting foreigners, have had the State Department declare certain premises temporary foreign missions. This convenient fiction allows the police to apply the 500-foot rule by which demonstrators are kept away from foreign embassies, and it gives the Secret Service the requisite authority to protect foreign dignitaries as they travel about the city or the country. It's handled on a low level, and it's routine.

But this time the visitor was Soviet Foreign Minister Eduard Shevardnadze, and the building being temporarily reflagged was the State Department. It was not the first time, but here the reflagging directly impinged (as the police intended) on a planned human rights demonstration. Barring a showing of real peril to Mr. Shevardnadze, it was a silly and wrong thing to do.

The State Department is not any old government building. It presents the official face of America to the world, and it is a reasonable and common place for demonstrations. The Shevardnadze visit last month came as many Americans were concerned to ensure that human rights issues are faithfully pursued. Sen. Helms is not one to flee from opportunities to harass the State Department. But on the floor of the Senate, he rallied 90 members to chide the department for declaring itself a foreign mission ''for the purpose of denying free speech to American citizens who planned to protest the tyranny of the Soviet regime.'' The department can say it acted in response to a routine police request, but no one seems to have thought of the spectacle of the leading nation of the free world muting the voice of Americans who wished to address peacefully an official of a totalitarian state.

We are confident that Mr. Shevardnadze, who represents a government much given these days to displays of glasnost, could have endured the waving of some placards, and that law enforcement officers could have done their usual first-class job and hustled the Soviet visitor safely from the curb to the State Department's front door, even as the placards waved