There are several methods - depending on how many you have, by hand with kalk, aiptasia x, lemon juice, or boiling water with a syringe have all been effective for me, I've found peppermint shrimp are more successful with smaller ones. Depending on your fish Berghia nudibranchs work really well, but some fish will go after them for a tasty treat.
The most important part with hand removal is making sure you kill and remove all of the anemone as a new one will grow from pieces left behind, and when threatened/you don't totally kill it, they will release babies into the water column and exponentially worsen your problem.
They can be tricky to fight, but if you are persistent you can win the war!