Words meaning to cancel or make ineffective are void, revoke, annul, counteract, offset, invalidate...
Two options are overturn and repeal.
it is sensation
stops and quits, but its more 4 letter word than 5, i just added plural and also abort
bail out
cancel
feckless
It is ineffectual (not inaffectual), and it means ineffective.
Forceless or Failing
cancel
If you mean "ineffective", that means it's not effective.
revoke! :)
Repeal
Exit Stop Close Discontinue
The word "cancel" is the opposite of sit, in the sense of convene.
Two options are overturn and repeal.
Useless is another word for ineffective. Additional synonyms include inadequate, powerless, futile and unproductive.
Another word for cancel is Nullify