The South Korean government plans to stop people sending balloons with anti-North Korean messages across the border.
The announcement came after Kim Yo-jong - sister of North Korean leader Kim Jong-un - said recent balloon-senders were "human scum".
For years, activists and defectors have sent messages criticising the North and its leadership across the border.
But the South Korean government said the balloons caused "tension".
One group told the BBC they had no plans to stop - and had another one million leaflets on order.
In the past, the balloons have also carried dollar notes, and even chocolate.
In 2014 North Korean soldiers attempted to shoot them down - leading to an exchange of fire across the border.
The North has also sent its own leaflets on helium balloons across the border in the past, demanding - among other things - an end to "further hostilities or stupid actions".
What did Kim Yo-jong say about the balloons?
Kim Yo-jong - who holds a powerful position in the North Korean government - issued a long statement on Thursday, blaming "defectors from the North" for recent leaflets.
"I wonder if the world knows what kind of riff-raff those foolish defectors are," said the statement, translated by KCNA Watch.
"Human scum - little short of wild animals who betrayed their own homeland - are engrossed in such unbecoming acts... they are sure to be called mongrel dogs as they bark in where they should not."
Ms Kim then said the "owners of the mongrel dogs" - that is, the South Korean government - should be held to account.
She threatened to scrap a no-hostility military pact, shut down the North-South liaison office, and withdraw from the joint-venture Kaesong Industrial Park.
How did the South react?
The South Korean government said it was planning legislation against the balloons, which they called "tension-causing".
"Actually, most of the leaflets have been found in our territory, causing environmental pollution and increasing burden on local people to get rid of them," said Ministry of Unification spokesperson Yoh Sang-key.
"Any act that could pose a threat to the life and property of those people should be stopped."
What do the balloon senders say?
Park Sang-hak, the chairman of Fighters for a Free North Korea, said they were undeterred - and had recently ordered another one million leaflets.
His group sent leaflets across the border 11 times last year, and three times this year. The latest was on 31 May - which led to Kim Yo-jong's furious reaction.
"We in South Korea have our own sovereignty, and in this democratic country people have three basic rights - and one of them is freedom of speech," said Mr Park.
"I, Park Sang-hak, am not living in Pyongyang nor am I a slave to the Kim family dictatorship. I am a South Korean citizen.
"The unification ministry has been trying to make this law for the last 15 years. Please go ahead. We are fine. We live in a 5G world now.
"If the leaflets get blocked, then we will send drones. They cannot stop us. Fact and truth cannot be stopped. The voice of 45,000 defectors who seek truth will continue."
'A tricky line to navigate'
This is not a good look. North Korea makes a demand - and hours later, South Korea complies.
These balloon launches have always been a sore point between the two sides.
Seoul is trying to re-establish ties with Pyongyang after a year of very little contact. There's even a push to amend a law which would allow North Korean firms to do businesses in South Korean territory.
The long-term logic of President Moon's administration is that engaging with North Korea will eventually lead to better human rights for its people.
But it's a tricky line to navigate.
South Korea is home to those who've escaped the North Korean regime. They promised them shelter. Most defectors have endured incredible hardship to win the right to free speech.
Pyongyang's statement described defectors as "human scum".
And by appearing to comply with Kim Yo-jong's demands so quickly, it may make some defectors wonder whose side South Korea is really on.