"Just call it a day" = "just hang it up."
If you normally work from eight to five, and at 4:30 there's a power failure, you might say, "Let's just call it a day." That is, it may not be a
full day, but it's the best you can do, so you might as well knock off and get a fresh start tomorrow. It's not a day (like a normal day) but you're going to call it one anyway.
The expression is often used figuratively. You're having major problems with your spouse and it doesn't seem possible to go on. "Let's just call it a day." (Let's not bother to try to complete this thing.)
In your quote, it would mean, "We may as well forget about the survival of the human race." (Kevin Costner will not be able to save us.)