Warren Buffett spent three hours answering questions live on CNBC's "Squawk Box" on Monday morning. After a Buffett marathon, we usually would look at his advice on what investors should do. This time, it's five key lessons on what investors shouldn't do:

1. Don't let world events (like what's happening in Ukraine) affect your investing decisions. Buffett said even if he knew a big war was unavoidable, "I will still be buying stock. You're going to invest your money in something over time. The one thing you can be sure of is if we went into some very major war, the value of money would go down. ... That's happened in virtually every war that I'm aware of. ... The last thing you want to do is hold money during a war. You might want to own a farm, you might want to own an apartment house, you might want to own securities. During World War II the stock market advanced. The stock market is going to advance over time."

Warren Buffett Lacy O'Toole | CNBC

2. Don't feel bad when stocks go down On a day that global stocks markets were reeling from worries that the Ukraine situation could lead to war, Buffett said, "When I got up this morning I actually looked at a stock on the computer, on the trades in London, that we're buying and it's down and I felt good. ... We were buying it on Friday and it's cheaper this morning and that's good news." Will he buy more? "Absolutely."

3. Don't think you have to be an expert to profit from stocks "The stock market just offers you so many opportunities, thousands and thousands of different businesses. You don't have to be an expert on every one of them. You don't need to be an expert on 10 percent of them even. You just have to have some conviction that either a given company, or a group of companies ... are likely to make more money five or 10 or 20 years from now than they're earning now. And that is not a difficult decision to come to." And if you have no expertise at all, Buffett recommends a low-cost index fund that tracks the . "Keeping costs to a minimum is enormously important in investing. ... If you're in effect paying out 1 or 2 percent annually of your portfolio, that's a big, big tax that you don't have to pay."

4. Don't go for the quick profit Asked if "activist investors" are really acting in the best interest of targeted companies and their shareholders, Buffett replied, "Generally speaking, they are interested in making a quick profit and there's no law against making quick profits. But our whole attitude in our own business and what we like to see with the businesses we own stock in is we want to run them for the people who are going to stay in rather than the people who are going to get out. At any given time, you can make more money, usually, selling the company. ... The answer isn't to sell the company. The answer is to keep running the company well. ... I could do certain things to jiggle up the price of Berkshire in the short run. It would not be good for the company over five or 10 years."

5. Don't put your money into bitcoins for the long run. "It's not a currency. It does not meet the test of a currency. I wouldn't be surprised if it's not around in 10 or 20 years. ... It is not a durable means of exchange. It's not a store of value. ... It's been a speculative—a very speculative—kind of Buck Rogers-type thing, and people buy and sell them because they hope they go up or down just like they did with tulip bulbs a long time ago."