IT IS NOT DIFFICULT to see the Irony in the Polish government's decision last Friday to officially ban the Solidarity Union. After all, since martial law was imposed in December, more than 600 Solidarity officials have been interned and the union's activities suspended. What, you could ask, was there left to outlaw?

Quite simply, General Jaruzelski wants to de-legalize the spirit and determination of the Polish people. Since December, Solidarity has continued to function underground, distributing leaflets, keeping the structure of the union intact, planning for the future. Although Solidarity was suspended, the hope was there that it might someday be allowed to resume its activities. Jaruzelski is trying to crush that hope.

But Poland's rulers are fooling themselves if they believe Solidarity will quietly disappear. On Sunday and Monday, thousands of men and women stopped working in the Gdansk shipyards to protest the ban. Fugitive leaders of Solidarity called Monday for a four-hour nationwide strike on November 10. Outlawed or not, the union can still be a painful thorn in Jaruzelski's side.

There is a feeling of impotence in the West with regard to what we can do to help Solidarity. Most actions--like economic sanctions against Poland or the Soviet Union--would serve no useful purpose and might even be counter-productive. President Reagan's decision to demote Poland from most-favored-nation status is no more than a symbolic gesture designed to appease the American conscience. Frustrating as it may be, there are no easy answers.

But we can take some comfort in the knowledge that Poles themselves are keeping the faith, even in the face of death. As Archbishop Jozef Glemp said Sunday in response to the banning. "We know that what is just, what is an ideal, cannot fall. Structures can be abolished, but no idea can disappear."