If I say to someone "do not X", I have (say) “prevented” them from doing X. If I say “you don't need to bother doing X”, how do I express saving them from the unnecessary work of doing X? Is there a better word than "prevent"?
relieve, alleviate, lighten, assuage, mitigate, allay
... mean to make something less grievous. relieve implies a lifting of enough of a burden to make it tolerable . alleviate implies temporary or partial lessening of pain or distress . lighten implies reducing a burdensome or depressing weight . assuage implies softening or sweetening what is harsh or disagreeable . mitigate suggests a moderating or countering of the effect of something violent or painful . allay implies an effective calming or soothing of fears or alarms .
The first word that came to mind when reading this question was "relieve", and I immediately searched (using the Google engine) for "relieve definition", and expanded the definition it provided to find this third definition, second section:
free someone from (a tiresome responsibility). "she relieved me of the household chores"
Of these alternative words with similar meanings, I still think "relieve" is the most appropriate in this case, because it is more like "free" and "lighten", I mean it is closest to the meaning of "lessening" the burden / duties of someone/people. But refrain from using the specific phrase "relieved them of / from (their) duty / duties" because that means firing employees.
Disclaimer: I'm merely providing my findings and sharing my knowledge and opinion, but I'm no expert on the English language, in fact quite the opposite - my vocabulary is very poor, or should be since I can use my fingers to count how many fiction / novels I've read in both my native tongue and English, and I'm too young to say otherwise anyway. Also, I don't use English often other than watching TV and reading programming manuals...
In light of the recent discovery of a previous answer proposing "exempt" (mentioned by Neil G and Kris, in the comments section), I believe it is the most appropriate choice as well. I didn't get to see that had been answered previously because it had been deleted before I answered, so I have no idea who gave that answer.
You have spared them unnecessary effort.
Verb, definition 3: to relieve of the necessity of doing or undergoing something.
For the second one?
saving them from the unnecessary work of doing X?
Discourage or maybe
unburden might fit. (it is used as disburden also)
to take a problem or burden away from (someone or something)
saved would be the correct word.
I have saved them from doing 'X'.