I’m surprised to hear that. Most balconies (in NSW, at least) are common property. In your case, you could politely request that the strata committee or manager tell them to not put their gear there, or tell them you will have it removed at their expense if they do it again or charge them “rent”. It depends on how much aggro you want to cause. But if it is your property, they have no “right” to access it.
In all likelihood the strata manager or SC don’t even know they are doing this and the abseilers don’t even realise that they need permission.