I guess this is the expected behavior, Spark will sync its sequence with the host if you have host activated.
If you want it to stop then play instantly when you send the command, you don't need the host button activated. You'll have to send your command right on the beat if you still want it to be synced up.
One technique that works fine for that is to assign a midi note to the play button, and have an ableton clip on your spark track that plays this note exactly at the moment where you want to start the Spark sequence.
But yeah, the Host function will make it so that Spark plays in time with the host.