Disabling Commenting on older posts, how?

Due to the massive influx of spam comments in the last few days (200-250 per day for me) I’ve turned off commenting altogether and plan on leaving it off indefinitely (yes, the spammers finally won as far as I’m concerned).

However there are several hundred older posts which still have the comment tick box enabled. I’m not about to go thorugh each one and turn comments off for each post, life is too short for that nonsense.

I’m wondering if there’s a way to turn off comments for past/older posts en masse? If not, it’s a feature that’s sorely needed.

I’ve already had to change my email address on the site so I don’t get flooded with comment approval emails.

Since spam comments have gotten so out of hand if there’s not a way to disable commenting on OLD posts (the main switch is set to off now for any new posts) I’m just going to abandon this section of my site entirely and start fresh with something that offers more options.