How can I stop the transaction log file (.ldf) from growing

If the log file has grown do to being in full recovery mode then set it to simple before going any further. This should immediately stop the log from growing.
 

Enterprise manager:
Right click on the database, properties, Options, set model to simple, OK.
 

t-sql
sp_dboption [dbname], 'trunc. log on chkpt.', 'true'