why LiAlH4 has no effect on double and triple bonds?

Neither LiAlH4 nor NaBH4 are able to reduce an isolated C=C bond. But if you have an enal (a conjugated aldehyde) it can react (as an electrophile) either at the β-carbon or at the carbonyle group's carbon. According to the HSAB Principle the β-carbon is a "soft" center and would react preferably with "soft" nucleophiles while the carbonyle carbon is a "hard" center and prefers to react with "hard" nucleophiles. Now, NaBH4 is a rather soft nucleophile and thus it reacts with an enal at the β carbon. After this reaction the C=C bond is gone. But, if what is left behind is still a very reactive simple aldehyde that gets reduced to the alcohol by NaBH4 in a second step. LiAlH4 on the other hand is a rather hard nucleophile and thus reacts with an enal at the carbonyle carbon. After this reaction there is just an isolated double bond left that can't be reduced by LiAlH4 in a second step.