This may differ state to state, but generally you're looking at a contractual issue. The body corporate will have an engagement document with the strata manager outlining what duties they will perform and how much they will be paid for it. What you're looking to do is to terminate this agreement.
Your options will fall along these lines:
1. If you have a valid basis for doing so, generally as set out in the agreement, you may be able to terminate the agreement - with or without notice depending on the circumstances;
2. Wait until the agreement term runs up and not renew it;
3. Seek to terminate the agreement without grounds to do so, and possibly bear a claim for damages from the manager for breach of contract; or
4. Seek agreement from the manager for a release from the contract (a 'rescission'), which may or may not come with conditions - and which will likely include some form of compensation for the manager's lost income.