Started conversion could be stopped. This requires passing CancellationToken to Start method.

var cancellationTokenSource = new CancellationTokenSource();
string outputPath = Path.ChangeExtension(Path.GetTempFileName(), ".mp4");
IMediaInfo mediaInfo = await FFmpeg.GetMediaInfo(Resources.MkvWithAudio);

IStream videoStream = mediaInfo.VideoStreams.FirstOrDefault()
    ?.SetCodec(VideoCodec.H264);

await FFmpeg.Conversions.New()
    .AddStream(videoStream)
    .SetOutput(outputPath)
    .Start(cancellationTokenSource.Token);

CancellationTokenSource can be cancelled manually...

cancellationTokenSource.Cancel();

or automatically after period of time:

cancellationTokenSource.CancelAfter(500);