Transmission RPC to PowerShell Object calls example. Removes torrents which are completed. I may make this into a tool when I get more time.

transmission_removecompleted.ps1

$transmission = Join-Path "C:\Program Files\Transmission" transmission-remote.exe
$server = "127.0.0.1:9091"

function GetTorrentObjectList {
    param (
        $TorrentList
    )
    $torrents = [Collections.ArrayList]@()

    foreach ($torrent in $TorrentList[1..($TorrentList.Count - 2)]) {
        $t = $torrent.TrimStart(' ') -isplit '\s{2,}'
        $torrents.Add([PSCustomObject]@{
            ID = $t[0]
            Done = $t[1]
            Have = $t[2]
            ETA = $t[3]
            Up = $t[4]
            Down = $t[5]
            Ratio = $t[6]
            Status = $t[7]
            Name = "$($t[8..9999])"
        }) | Out-Null
    }

    return $torrents
}

$torrentlist = & $transmission $server -l

$torrents = GetTorrentObjectList $torrentlist

$torrents | Where-Object { $_.Done -eq '100%' } | ForEach-Object {
    Write-Host ('Removing Torrent by ID: {0}' -f $_.ID) -ForegroundColor Red

    & $transmission -t $_.ID --remove
}