Automatic cut off after switch is pressed
That's simple &mdash; you just need to AC-couple the switch, like this:







simulate this circuit &ndash; Schematic created using CircuitLab

R4 and C4 are new components; their time constant is set to 10 ms, a small fraction of your timer period. Regardless of how long the switch is held, the TRIGGER input of the 555 will only see a short low-going pulse.