The people of the U.S. have decided that Social Security it necessary because the nation existed without it and a huge amount of people were starving and freezing to death once they became elderly. Churches, civic organizations, and charities were unable to handle the situation. That being said Social Security is already strictly voluntary; if you don't agree with the law you are allowed to leave and go to a place where it does not exist.
The argument about using force is a false argument, and is exclusively used by morons. There is a law in place requiring Social Security to be funded through contributions by members of society. If you disagree with this and don't pay you are breaking a law. Generally you are tried, found guilty and fined a specific amount depending on the level of your infraction. If you refuse to pay the fine, or are unable to pay the fine the court can take money away from you in the form of liens on property, or forfeitures. The only time violence comes into it is if you repeatedly refuse to follow the law. You would have to refuse to obey the law by not paying your S.S. taxes, refuse to follow the law by not paying your fine, and refuse to follow the law by disobeying the police when they came to arrest you. At this point there might be violence because you are refusing arrest and putting the police in a dangerous situation, but at no time are you under the threat of violence to make you pay taxes. To argue this is idiotic at best, and borderline retarded at worst.
As for your original question about what to do to fix Social Security the answer is simple. Remove the ceiling on what people contribute. So a rich person, instead of only having to contribute money up to a certain level of income, and then being exempt for the rest of their income simply pays Social Security taxes on all of their income like the majority of people in the country. That alone would solve the problem.
Even without that the program will pay out 100% of obligations for almost the next 30 years, and about 70% of obligations into the foreseeable future after that with absolutely no changes. If I could show you 1 private company that was guaranteed to be able to provide this security for over 30 years you would be pointing to it as the greatest example of free market triumph in the history of the world.