I'm not sure, but I think if you do a full backup of the database, then you will be able to shrink it. If not, try changing the recovery model to simple, then do a full backup, and try shrinking it again.
Jguen is right, the best process for shrinking the log file is to set the recovery model to simple. You will then need to run a shrink on the database after changing the recovery model.
The recovery model can be set in management studio by right clicking on the database name, then go to Properties > Options > Recovery Model.
The shrink can be done by right clicking on the database name, then go to Tasks > Shrink > Database. You can also choose the "Files" option, then specify to only shrink the log file if you truly only want the log file to shrink.