(CNN MONEY) Mark Zuckerberg has always encouraged Facebook employees to think for themselves, but one issue is trying his patience.

People have been crossing out "black lives matter" on the walls of Facebook's headquarters and writing "all lives matter."

Zuckerberg wrote a strongly worded memo to employees earlier this week about "several recent instances."

In his note, the Facebook (FB, Tech30) CEO seemed frustrated by the fact that the act has continued, despite making it clear in the past that it was "unacceptable."

"I was already very disappointed by this disrespectful behavior before," Zuckerberg wrote. "But after my communication I now consider this malicious as well ...This has been a deeply hurtful and tiresome experience for the black community and really the entire Facebook community."

Black lives matter sprung to life as a hashtag in 2012 after the death of Trayvon Martin. The phrase went viral on social media, drawing people into a conversation about police brutality and inequality, and unifying thousands across the country on these issues. Some people pushed back with the slogan "all lives matter."

"But when someone says ALL lives matter, it can sound like that person is dismissing the specific pain behind the slogan," CNN's Donna Brazile wrote last year. "Those who are experiencing the pain and trauma of the black experience in this country don't want their rallying cry to be watered down with a generic feel-good catchphrase."

Zuckerberg gave his own interpretation of the movement: "'Black lives matter' doesn't mean other lives don't -- it's simply asking that the black community also achieves the justice they deserve."

Some walls of Facebook's offices are covered in whiteboards in chalkboards. Usually adorned with signs reading, "write something," they're covered in layers of messages, doodles and signatures from employees and visitors.

"Crossing out something means silencing speech, or that one person's speech is more important than another's," Zuckerberg admonished.

A Facebook spokeswoman confirmed that the memo had been sent but would not tell CNNMoney if those responsible for the acts had been identified or disciplined.

Here's the full text of the memo, obtained by Gizmodo:

"There have been several recent instances of people crossing out 'black lives matter' and writing 'all lives matter' on the walls at MPK.

Despite my clear communication at Q&A last week that this was unacceptable, and messages from several other leaders from across the company, this has happened again. I was already very disappointed by this disrespectful behavior before, but after my communication I now consider this malicious as well.

There are specific issues affecting the black community in the United States, coming from a history of oppression and racism. 'Black lives matter' doesn't mean other lives don't -- it's simply asking that the black community also achieves the justice they deserve.

We've never had rules around what people can write on our walls -- we expect everybody to treat each other with respect. Regardless of the content or location, crossing out something means silencing speech, or that one person's speech is more important than another's. Facebook should be a service and a community where everyone is treated with respect.

This has been a deeply hurtful and tiresome experience for the black community and really the entire Facebook community, and we are now investigating the current incidents.

I hope and encourage people to participate in the Black@ town hall on 3/4 to educate themselves about what the Black Lives Matter movement is about."