When Sears, Roebuck & Co. and International Business Machines Corp. started their on-line computer network known as Prodigy, they saw it as a means for customers to instantly review stock quotes, buy airline tickets or even send messages to one another. The companies probably never dreamed that their network would someday be used against them.

But now that a group of angry Prodigy subscribers has tried to foment revolution on-line, Prodigy officials have found an efficient way of fighting back. This week the firm unplugged about a dozen outspoken dissidents whom it says were pestering innocent users with the electronic equivalent of junk mail.

But what Prodigy sees as a way to stop needless harassment seems to others as a blatant example of censorship. That's because the people bumped from the Prodigy system included the most active critics of a planned price increase for Prodigy's electronic mail service.

Using electronic mail on the network, the dissidents had urged other subscribers to join the revolt by boycotting the advertisers that buy time on Prodigy's network.

"Prodigy is arguing they don't want people harassing their users," said Gary Arlen, editor of Interactivity Report, a Bethesda newsletter that follows the on-line industry.

"I think that's a stretch. It's a way to keep their advertisers pleased."

The incident is the latest to spotlight the difficulties society faces as it struggles to adapt old laws and customs to emerging electronic networks.

Electronic networks offer users new means of communicating, advertising and buying goods -- serving in some respects as substitutes for telephones, television or newspapers. But because such services are a hybrid, few analogies apply neatly.

Some people say on-line services should protect the right of all expression, as a phone system does, while Prodigy argues it is more similar to a newspaper, which is free to publish what it chooses.

Prodigy's troubles began two months ago when it announced that households would be able to send their first 30 electronic mail messages free but would get charged 25 cents for each additional message. A core of angry subscribers first protested by posting notices to Prodigy's on-line bulletin boards, the computer equivalent of neighborhood kiosks. Prodigy said it posted thousands of such complaints for others to read -- but it didn't publish them all.

When the writers urged a boycott of Prodigy advertisers -- firms selling products on the network -- Prodigy's editors returned the messages to the senders.

"We're not going to post something designed to destroy our business," said Geoffrey Moore, Prodigy's director of market programs and communications.

Moore likened the decision to a newspaper rejecting a letter to the editor, or rejecting an advertisement that criticizes the newspaper's largest advertisers.

This week Prodigy decided enough is enough and refused to post any more messages about the rate increase. But what especially angered officials was when the dissidents innundated other users with electronic chain letters urging them to join the protest and boycott. Moore said users complained, so Prodigy bumped the offenders.

And now the protestors say that's unfair.

"We're not being abusive. We're not being vulgar. All we're doing is making our {opinions} known," said Larry Wienner, 22, a Prodigy user from Randallstown, Md.

Wienner said the bumped dissidents are so hooked on Prodigy that they may try to re-subscribe under assumed names.