Clear-RecycleBin -Force
Example
PS C:\Users\Chris> Clear-RecycleBin Confirm
Are you sure you want to perform this action?
Performing the operation "Clear-RecycleBin" on target "All of the contents of the Recycle Bin".
[Y] Yes [A] Yes to All [N] No [L] No to All [S] Suspend [?] Help (default is "Y"): y
PS C:\Users\Chris>