Are you worried about your sub stealing your client or are you trying to keep your client from dumping you to go with the sub? If it is the former, hire a better class of sub. If it is the latter, do you really want to work for slime?
I never hire a sub without the client's explicit approval. I always discuss the project and the client with the sub prior to committing. I've never had a problem. Several times, we've reached milestones in projects where I was the one that wasn't adding enough value and I would bow out in favor of the sub--I've never been hurt by that, my clients see it at caring about their costs and tend to increase my work with them.
If everyone in a project has the same integrity, then using subs can really leverage your time. If anyone on the project is lacking in integrity then they should be killed and buried under the pilings (or at least black balled). I figure if the client and/or the sub are slime I'm better off being out of it no matter what they're paying.
David