If you have assigned the policy directly to the IAM user, then the Unregister-IAMUserPolicy cmdlet can be used:
PS C:\>Unregister-IAMUserPolicy -UserName Ramesh -PolicyArn arn:aws:iam::aws:policy/PowerUserAccess
If you want to detach all the policies directly assigned to the user, then use Get-IAMAttachedUserPolicies, and pipe it to Unregister-IAMUserPolicy for that specific user:
PS C:\>Get-IAMAttachedUserPolicies -UserName Ramesh | Unregister-IAMUserPolicy -Username Ramesh