Since acyclovir can help treat viral conditions and prevent the spread of the viral infection in your body then yes Acyclovir can help prevent the flu before it takes full effect on your body.
However Acyclovir and nothing else can actually treat or fully cure the flu once you have it fully in your body.
Acyclovir can also be used to treat other viral infections such as genital herpes, cold sores, shingles, and chicken pox.