Banning them from the store/crippling their existing installs is basically: "we don't like you competing with our first party offerings, so get the hell off our platform"

Suing them now, given that Kik is still active on other platforms, is basically: "we don't like you competing with our offerings, so we're going to prevent you from operating anywhere, even on platforms we don't own or have any control over"

I don't know about you, but the second seems a bit more evil than the first.