Spacebar will just center the screen on your champ, but you'd have to hold it down to leave it centered. So it's not really a toggle for locked screen, and more just a snap-to-champion feature.
I honestly have never made this transition myself, as unlocked feels far more natural for me (I was super frustrated even playing the tutorial when I first started playing, until they showed how to unlock the camera). That said, I feel like you'd pretty much just have to force the change on yourself and get used to it.
Maybe just play a few ARAMs or even co-op games at first, just to get used to having to control the camera yourself. Co-op is good cause it doesn't really matter if you screw up a couple times because of it. Should still win the game regardless. ARAM tends to require less screen scrolling, because the action is linear.
Also, be sure to play with the scroll speed slider in the settings, too. If it's set too high for your preferences, it's easy to suddenly feel out of control. Maybe starting with a scroll speed more in the middle and slowly working up to what feels comfortable would help?