Yes, you can clear Windows EventLog one by one from Server Manager > Diagnostics > Event Viewer but it there are a lot of event log to clear right? instead of manually clearning the logs you can use powershell command. Open up your powershell command prompt and use this command :

wevtutil el | foreach { wevtutil cl $_ }

If you want to know what is being cleared you can add additional command like this :

wevtutil el | Foreach-Object {Write-Host "Clearing $_"; wevtutil cl "$_"}

Hope it will be useful your you. Cheers!

References :
blogs.msdn.com/b/emberger
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