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.
unburden might fit. (it is used as disburden also)
to take a problem or burden away from (someone or something)