Well countries can prevent their citizens from leaving and they do this all the time - such as people with arrest warrants, outstanding debts, kids (without permission from both parents), etc

But it would be unlikely that a country would prevent all of their citizens from being able to leave - this would not go down well either with it's own citizens or with the international community. But, in theory, there's nothing to prevent them from being able to do it if they could get the government to approve it and convince either the police or military (or both) to actually enforce it.

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But if you were in another country when a law like this was passed than they wouldn't automatically be able to force you to return because you would be bound by the laws of the country you are in, not the laws of the country you left. But they could apply to the country you are in to have you extradited back.

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And would matter if it is part of EU, for example?

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One of the "rules" about being in the EU is that they allow freedom of travel between member countries so no country that was a member of the EU would be allowed to pass a law banning this without first leaving or getting kicked out of the EU.