Which make and model?  They have aftermarket devices you can plug-in to disable it.  To my knowledge (someone correct me) you cannot disable it by pressing some button in your car unless you go into some Mode (Sport, Manual, etc.) - but then you have to stay in that mode.  But all this only disables it via software;  the hardware is still intact and in my mind a liability still, but replacing the hardware (cam and lifters plus an ECU re-tune) is the true full-proof solution can cost $3,000+.  Either option will void your warranty if it’s still under it.

That’s why for long term reliability and peace of mind I always advise people not to get a vehicle that has cylinder deactivation in the first place.

Have not seen a button to disable similar to "auto start/stop". I really like the idea to read the owners manual, there may be a trick listed. Be wary of aftermarket devices, they may cause unitended consequences that could be costly. Look inside yourself and ask which would give you peace. Having a car already paid for, know the quirks, or maybe spending more for an auto without cyl deact, but being happy?

@DayWalker is correct as usual. I've only done them the hard way so I've not tried any of the OBD-II options. But you can do it. It just costs quite a bit because you have to open up the motor.

yes

have you checked your owners manual?

Cylinder Deactivation Deactivation.