To turn off the arpeggiator, simply click one of the Assign Mode buttons, like Poly 1, for example.
Here's what the help documentation says:
A unique aspect of the Jupiter-4 design is that the arpeggiator does not have an "on/off" switch, per se. Instead, the white Arpeggio section direction buttons interact with the blue Assign Mode buttons - only one of these buttons is active at any time, so pressing any of the Arpeggio or Assign Mode buttons disables any other currently selected mode buttons. At any given time, Mercury-4 is either in a "live play" mode (one of the blue Assign Mode buttons pressed), or one of the Arpeggio direction buttons is pressed, enabling the arpeggiator.
Learn more here: Mercury-4 arpeggio documentation.