Combine multiple logical operators

You want to use this:

where {$_.user.tostring() -notin ("USER1","USER2","USER3")}

Or this:

where {($_.user.tostring() -ne "USER1") -and ($_.user.tostring() -ne "USER2") -and ($_.user.tostring() -ne "USER3") }

That's really about as simple as you can get. The boolean operators should, in general, only be used to combine comparison operators (or other things you know represent boolean values).

Your code here:

where {$_.user.tostring() -ne {"USER1" -or "USER2" -or "USER3"}}

This is basically nonsense. It's always going to evaluate to true. {"USER1" -or "USER2" -or "USER3"} is of datatype ScriptBlock.

PS C:\> ({"USER1" -or "USER2" -or "USER3"}).GetType().FullNameSystem.Management.Automation.ScriptBlock

I believe PowerShell will cast this to a string, but even if it gets cast to a string, it's still not evaluated as a boolean expression:

PS C:> ({"USER1" -or "USER2" -or "USER3"}).ToString()"USER1" -or "USER2" -or "USER3"

That's going to evaluate to True unless the user is literally '"USER1" -or "USER2" -or "USER3"'

If you changed it so that it was a parenthetic expression instead of a script block:

where {$_.user.tostring() -ne ("USER1" -or "USER2" -or "USER3")}

Then it would always be true. ("USER1" -or "USER2" -or "USER3") is of type Boolean, and will always have the value of true.

PS C:\> ("USER1" -or "USER2" -or "USER3")True

So you're basically running:

where {$_.user.tostring() -ne $true }

Again, even if PowerShell converts everything to a string like I think it will, it's pretty unlikely you've got a user named "True". So this will always be true.

Assuming your dealing with a array it can be done in this similar fashion you wrote it, even shorter check this out:

PS Z:\> @("food","drink","cheese","dog") -match "(food|cheese)"foodcheesePS Z:\> @("food","drink","cheese","dog") -notmatch "(food|cheese)"drinkdogwhere clause:PS Z:\> @("food","drink","cheese","dog") | where {$_ -notmatch "(food|cheese)"}drinkdog