First of all, a high spot price is not in itself a problem as long as the high price lasts only for a short time and effectively brings needed capacity online. Where you have a serious problem is when spot prices spike, no one responds and load-shedding starts. I appreciate that SA has actually got to the point of load-shedding several times now, which is indicative of a severe problem.
However, saying that renewables are the cause of this is the same as saying that nuclear is the cause of Chernobyl. Nuclear plants don't inherently melt down if they are properly designed and operated and renewables don't inherently cause load-shedding if they are integrated to a properly planned and operated bulk power system.
What is occurring in SA is a power system planning and market design failure that is not inherent to any one technology.
It seems the system operator has been relying on reliability services provided by large synchronous generators without actually pricing and creating a market for those services, which has caused them to disappear with coal retirements and leave the power system in a chronically insecure state. In order for the market to ensure both day-ahead generation adequacy and as well as real-time operating reserve margin for unit contingencies, generators need to be able to get paid for providing those services, at a level that makes it economical to invest in new assets or maintain existing ones.
This is a policy problem, not a technical one.
FWIW I work for the company that owned the now decommissioned Hazelwood coal plant in Victoria, but I'm not involved with that plant at all.