All of the above is true, but there is another way. Only in certain situations, but sometimes you can just ignore the curser completely. If you have a heavy trasher and a Tfb that doesn't care about cost, you can get all of your starting junk out of the way and then every curse your opponent hands you is a $5 card. For instance, if you open double forager in a game with mountebank, you will be guaranteed cards to trash. This is just a really simple example, but it can get more complex.
Sometimes it's awesome to see your opponent trying to sea hag you and you have a very trim deck that draws itself and a trash for benefit that doesn't care about cost. They're feeding you good cards! This is rare, but it's important to note that usually the best defense against cursers is to curse back, and sometimes the best defense is to do something else entirely, but either way, you best trash your curses! If you can't trash your curses, there can still be other ways.
In general if your opponent is going heavy on cursers and you know you won't win the curse split you have to look at what he's missing out on by buying these cursers. Venture is a great way to overcome a no +action no +buy game with cursers because venture doesn't care how purple and green your deck is. Upgrade can be an okay way too, but only with other support. Rebuild is a great option. It doesn't get rid of your curses, but it does improve every other worthless card in your deck. Pretty much any alt VP can handle curses better than a straight buy-all-the-provinces strategy.
Ambassador? Sometimes even with no cursers you will want to actually BUY a curse in an ambassador game.
In general, when combating curses, know what's in your deck, think of creative ways to either remove the curses from your deck, turn them into something useful, or give them back to the guy that gave them to you. If you can't do any of those things, then just buy those cursers and hope for the shuffle luck/first player advantage.
Also know which cursers are better. If young witch, mountebank and witch are all out, don't buy a young witch at four, and pass up the witch for a mountebank at five. And above all, don't fall into the trap of buying a potion just to get a familiar. If there are no better reasons for buying a potion, just find a way around your opponents familiar strategy. It will be faster.
I'm not an expert, but I play an insane amount of dominion. These are my opinions.