Stops an appPool and waits until it has entered the stopped state. By default the poll period is 1 second. This commandlet may take an AppPool object from the pipeline.

Stop-WebAppPoolSafely.ps1

Import-Module WebAdministration

function Stop-WebAppPoolSafely
{
       <#
     .SYNOPSIS
     Stops an appPool and waits until it has entered the stopped state
     .DESCRIPTION
     Stops an appPool and waits until it has entered the stopped state. By default the poll period is 1 second. This commandlet may take an AppPool object from the pipeline.
     .EXAMPLE
     Stop-WebAppPoolSafely "APP_POOL_NAME"
     Stops an appPool by name.
     .EXAMPLE
     Stop-WebAppPoolSafely -appPool $appPool
     Stop the appPool object.
     .EXAMPLE
     $appPool | Stop-WebAppPoolSafely
     Stop the appPool from the pipeline.
     .EXAMPLE
     $appPools | Stop-WebAppPoolSafely
     Multiple appPools may be passed in through the pipeline.
     .EXAMPLE
     Stop-WebAppPoolSafely "APP_POOL_NAME" -poll 5
     Stop an appPool with a poll period of 5 seconds.
     .EXAMPLE
     Stop-WebAppPoolSafely "APP_POOL_NAME" -verbose
     Stop an appPool with staus updates
     .PARAMETER appPoolName
     The name the appPool to stop
     .PARAMETER appPool
     The appPool to stop
     .PARAMETER pollPeriod
     The time period to wait between checking appPool status
     #>
    [CmdletBinding()]
     param(     
        [Parameter(Position=0, ParameterSetName = 'name')]
        [string]$appPoolName,    
        [Parameter(ValueFromPipeline=$true, ParameterSetName = 'appPool')]
        $appPool,  
        [Parameter()]      
        [int]$pollPeriod = 1   
     )

    process
    {  
        if ($PSCmdlet.ParameterSetName -eq 'appPool')
        {
            $appPoolName = $appPool.Name
        }

        if ( (Get-WebAppPoolState -Name $appPoolName).Value -eq "Stopped" )
        {
            Write-Warning "AppPool already stopped: $appPoolName"
            return
        }

        Write-Verbose "Shutting down the AppPool: $appPoolName"
        $currentState = (Get-WebAppPoolState $appPoolName).Value
        Write-Verbose "$appPoolName is $currentState";

        # Signal to stop.
        Stop-WebAppPool -Name $appPoolName;
        do 
        { 
            $currentState = (Get-WebAppPoolState $appPoolName).Value
            Write-Verbose "$appPoolName is $currentState";
            Start-Sleep -Seconds $pollPeriod 
        }
        until ( (Get-WebAppPoolState -Name $appPoolName).Value -eq "Stopped" )

        $currentState = (Get-WebAppPoolState $appPoolName).Value
        Write-Verbose "$appPoolName is $currentState";
    }
}

Credit to @jdpoligo