remove user from all sitecollection using powershell

Hi All,

I am trying to remove one user from all site collection. I am trying to execute below powershell script

Get-SPSite http://SPSD:8083 |Get-SPWeb |Remove-SPUser "demo\user1"

but i am getting below error

Remove-SPUser : You must specify a valid user object or user identity.

Thanks

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Regards,

Ajit Shinde