Cleanup Error Messages in SQL Server ERRORLOG
I'm not sure I understand your motivation behind wanting to Cleanup Error Messages in SQL Server ERRORLOG.  You can certainly close the current file and create a new by either:


Restarting SQL Server


OR


Executing sp_cycle_errorlog


Keep in mind that the SQL Server error log is a file that is full of messages generated by SQL Server. By default this tells you when log backups occurred, other informational events, and even contains pieces and parts of stack dumps. In short, it’s a treasure trove of information. When SQL Server is in trouble, it’s nice to have this available as a source of information during troubleshooting.

SQL Server keeps up to 6 error log files around by default. You can easily change this. Open up your copy of SSMS and:


Expand the “Management” folder.
Right click on “SQL Server Logs”
Select “Configure”
Check the box “Limit the number of error log files before they are
recycled”
Pick some value to put in the “Maximum number of error log failes”
box
Click “OK”


Check out Forgotten Maintenance – Cycling the SQL Server Error Log for more information about configuring the SQL Server ERRORLOG