You're right, it is imposible to make risk zero. However you can optimize your risk, ensuring that you get the best posible return for a given risk. That's the whole point of building a portfolio or using a diversity of trading strategies... not putting all your eggs in one basket.
there is always risk as no probability factor is 100%, BUT there are ways of reducing risk, which can lead to ALMOST eliminating it. Theoretically speaking risk can not be eliminated entirely. Very much depends on the time frame used to reduce risk, the shorter the time you have to make money, generally the more the risk. That you can relate to any regular business.
Yes. By becoming a market maker and trading against the order flow you will incurr zero risk. You will profit by at least the spread on every trade. Your stop loss is the order you have in hand which can be filled at break even at the worst case.
Yes. The "definition" of risk free is the 3-month (US) Treasury bill, now yielding over 5%. Some would argue even that is not truly risk free.
You can manage other people's money. Then your risk is zero as you still get paid even if you lose money. The catch of course is that the people who have invested in you have taken the risk.