Hi there best way is to
1) dbcc opentran(datbasename) - this will show the oldest open transaction
2) Kill this process
3) backup log Database name with no_log
4) use databases name
5) dbcc shrinkfile(databasename_log,100) - shrinks the log back to 100mb
then backup the database is full mode as it will be had to recover if you keep it backed up with nolog.
Regards,
Terry