“Our task is the protection of socialist achievements. … Those who do not respect our borders will feel the bullet.” (1970s Socialist Unity Party speech to Berlin Wall guards – video excerpts shown in The Secret Life of the Berlin Wall)
The Socialist Unity Party meant that people who disrespected socialism by trying to escape it were to “feel the bullet”. So I guess there’s no point telling modern socialists that it’s socialist to make people who do not respect borders “feel the bullet” – or even just the hardness of Trump’s wall. They would reply that trying to prevent people from entering your country is ‘not real socialism’ – and they’d be absolutely right. 
The Berlin Wall fell in November, 31 years ago.