In order to capture a group, you need to deprive it of all "liberties," the empty spaces touching the stones. In this case, the southwest group has two liberties, on A1 and C2.
However, as the rules don't allow you to leave any of your own stones without liberties at the end of your turn, you can't play on A1 due to suicide, and the same goes for C2. In order to capture that group, you'd have to somehow play on A1 and C2 at the exact same time, but you only get to play one stone per turn.
So how do you capture the southwest group? The answer is, you can't! Welcome to an important fundamental part of Go strategy: "Life and Death."
The southwest group is in a state where it is invincible, and nothing you do can capture it, so we refer to this group as "alive." On the other hand, a group which can't save itself from being captured is "dead."
How does a group become alive? In the exact way shown on your board here. Inside the group are two separate liberties (empty spaces), and since you can't play inside both spaces at the same time, capturing the group is impossible. Liberties inside a group are called "eyes," and if a group is capable of making two eyes, it is alive.
A group that can't save itself is a group that has no way of making two eyes. These groups are considered dead. Typically Go players do not even go through the process of capturing groups that are clearly dead, and instead just capture the dead stones once the game is over.
Though, playing out the position to check if the stones are really dead does not affect the final score, as each additional stone added to the dead group is worth a point for the capturing player, but each additional stone played to complete the capture fills up one point's worth of the capturing player's territory, so it all balances out in the end.