It sounds like you may of set your Premium SMS spend control to $0.00. Now you can definitely set your own limit. It really depends on what type of services you're using. If for example you want to enter a competition, you can set your limit to say $5.00.
Had you recently contacted us in regards to subscription charges that you hadn't actually opted in for? We've got a handy thread here which should be of some help to you → https://yescrowd.optus.com.au/t5/Online-Mobile-Self-Service/Premium-SMS-PSMS-and-How-To-Set-A-Spend-....
Let us know how that goes Sarah. We're happy to help if need be.