How do you stop the alarm on steamworks island?

Answer 1

You don't. You have to go up to the dome, to the far left and use the mallet on the crack to break the glass.

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Once you use the herbicide and reach the Maintenance Closet, taking the Living Quarters key sets off the alarm, so you cannot go out the way you came it. Hopefully you have the Rubber Mallet from the junk bin at Sully's. If not, return the key to the robot, leave, and come back with the mallet.

Once you have the key, climb up the the top of the dome, and you will find the glass is cracked and weak on the left side. Use your mallet to smash the glass, and exit.