Yep. A legal one though.
1. Reply STOP to any SMS you get
2. Contact Optus and request your Premium SMS limit be set to $0 (prevents it happening again)
3. Request they dispute the charges.
About half of Optus customer service will say yes, the other half might try ask you to do it yourself. They are required to raise a dispute if requested. You might need to contact back again to get the right 50%.
In general the mony will be recreditted back on your next bill.