A. There is no way to truncate the errorlog or switch it either, short of stop/starting the MSSQLSERVER service.
If the errorlog is filling up your disk then your choices are :-
- Fix the application(s) that are causing the errors.
- NTFS compress the log directory to reduce the physical space it takes
- Move the errorlog to another directory and use the -e startup option to point the errorlogs to the new place.