Easy way: disconnect the negative battery cable for half an hour. However, you lose your radio settings and such.
Second way - this works on a 2nd Gen Legacy (95-99) and I think on a 1st gen as well, so it might work on yours.
Under the hood, there's a second fusebox behind the battery and washer fluid reservoir. Open the box, and remove the second green fuse from the left (labeled SBF #2 - slow-blow fuse - in the diagram on the box lid) for half an hour. No loss of radio settings or anything. Your transmission and engine management systems will have to re-learn, but sometimes that's a good thing.