You can use this script for remove IP Adresses from dhcp server. Removing IP adresses is important when you have a scope with subnet musk and you have more mobile devices. by this script you can remove mobile devices easily. By the way while deleting mobile devices, it keeps rezervations IP adresses.
What is DHCP: 

DHCP (Dynamic Host Configuration Protocol) is a protocol used to provide quick, automatic, and central management for the distribution of IP addresses within a network.

DHCP is also used to configure the proper subnet mask, default gateway, and DNS server information on the device.


$message = New-Object -ComObject 
$dhcpServer = Read-Host "DHCP Server IP / Name" 
$scopeId = Read-Host "DHCP Scope Address for Remove IP Addresses" 
# select ip adresses which are not rezervation and are android, Apple Iphone or galaxy mobile 
$ipList = Get-DhcpServerv4Lease -ComputerName $dhcpServer -ScopeId $scopeId | where {($_.HostName -like 'android*' -or $_.HostName -like '*phone*' -or $_.HostName -like '*galaxy*'-and $_.AddressState -NOTlike '*Reservation*'} 
Write-Host "Count of will Remove IP Addresses" 
Write-Host "Android Count: " + ($ipList | where {$_.HostName -like 'android*' -or $_.HostName -like '*galaxy*'}).Count 
Write-Host "IPhone Count: " + ($ipList | where {$_.HostName -like '*phone*'}).Count 
Write-Host "Total: " + $ipList.Count 
$confirm = $message.popup("IP Adresses will remove from DHCP, are you sure?",0 ,"Remove IP Adresses", 4) 
if ($confirm -eq 6) 
    $ipList | Remove-DhcpServerv4Lease -ComputerName $dhcpServer 
    $message.Popup("IP Adresses removed.", 0, "Info", 0)