So there is a way to throw the current transaction log away, but it is not a step to take lightly. This will obviously break the log chain, so before this you do this, you should run DBCC CHECKDB WITH EXTENDED_LOGICAL_CHECKS to make as sure as possible that you don't have any corruption. I would also recommend that you run this on a restored backup to be more certain.
The way you do it is that you create a database snapshot. You can do some operations in the source database, but nothing you want to keep. To wit, next step is to revert from the snapshot. You do this with RESTORE DATABASE, saying FROM DATABASE_SNAPSHOT rather than FROM DISK. When you restore the snapshot, the log will be thrown away, and you will get a transaction log of half a megabyte. Which you obviously you should grow to the desired size immediately.
I would strongly recommend that you first try this operation in your dev environment.