Get-WinEvent -Path testfile.etl -Oldest
Remove-Item -Force testfile.etl

Testfile.etl should be deleted

Produces an error that the file cannot be removed because it is being used by another process. Closing PowerShell is the only way to release the lock and then delete the file. 

Name                           Value                                                                                   
----                           -----                                                                                   
PSVersion                      5.0.10586.117                                                                           
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}                                                                 
BuildVersion                   10.0.10586.117                                                                          
CLRVersion                     4.0.30319.17929                                                                         
WSManStackVersion              3.0                                                                                     
PSRemotingProtocolVersion      2.3                                                                                     
SerializationVersion           1.1.0.1