In a letter sent by Foreign Minister Alain Aimé Nyamitwe, the Burundian government said the three UN investigators - Pablo de Greiff from Colombia, Christof Heyns from South Africa and Maya Sahli-Fadel of Algeria - were declared persona non grata with immediate effect.
With escalating violence and the rejection of outside intervention, Burundi has become increasingly isolated diplomatically itself in recent months.
About a year ago, the African Union threatened to send thousands of soldiers, to intervene in the country - an intervention authorised without the consent of Bujumbura.
"It's a message that essentially says 'We called your bluff, on international intervention and right now, we're not scared and we're willing to be assertive, and we're willing to be even agressive vis-à-vis Western donors and vis-à-vis the UN'," political analyst Alex Fielding told RFI.
Since December 2015 the African Union and heads of state have avoided confrontation with the Nkurunziza government, he said. "Since Burundi called their bluff, the international community has been very limited in the credibility of its threats because it simply doesn't have the capability to follow through on them and that has emboldened the government in a series of ramped up measures of rhetoric and actions against the UN and against the West that has now led to this banning of the three investigators."
Other countries and international bodies seem reluctant to put pressure on Burundi, analysts say.
"I think everyone isn't really at ease, trying to mediate or even discipline a bit Nkurunziza regarding what he's doing at home while many leaders around Burundi are trying to do exactly the same thing, meaning, staying for one more term, and thinking maybe already about how they're going to quell any opposition coming up," Benjamin Chemouni, an Africa researcher at the London School of Economics, says.
"Of course, the international community could step in, but now it should be clear that sending three observers, just to investigate might not be enough. It really puts into question whether we need this or maybe we need a much wider and kind of systematic presence of the UN."
Now, it seems as if there is a lot of talk but no real action to stem the violence in Burundi.
"The only thing that's most important now, the only card to play at the moment is to try and convince the neighbouring countries to put pressure on Burundi,"Thierry Vircoulon, senior researcher at the International Crisis Group, told RFI.
He says that, while the international community needs to intervene, Burundi's neighbours should be leading the way.
"The east African community organised another useless summit in September where it discussed the situation in Burundi but it basically took no action, so now the only card left is to try and have them take sanctions against the Burundian regime."
LSE's Chemouni agrees.
"Another solution as well is about freezing assets and denying visas to some people of the regime in Bujumbura to travel in the region," he says.
"Those people have assets in South Africa, Kenya, and if there were a coordinated response for them not to be able to travel anymore and not being able to stash money in African banks, that's something they could listen to because in a way, this removes their exit strategy from Burundi."
Alex Fielding concluded that between the lack of political will from leaders close to Burundi, and the lack of real mechanisms to effectively check President Nkurunziza's actions, major challenges lay ahead.
The Burundian government did not respond to interview requests.