The guns stopped them.
3 seconds sound clip from the Aliens (1986) movie soundboard.
You can hear this line at 01:42:22 in the Blu-ray version of the movie.
Quote context
[...]
- Next time they walk right up and knock.
- Yeah, but they don't know that.
- They're probably looking for other ways to get in. That'll take 'em a while.
- Maybe we got 'em demoralized.
[...]