The Kremlin's response was short to follow. They interpreted this move as hostile, and the Russians said they retained their right to take reciprocal measures, which is of course a thinly veiled threat. Their diplomats said in politics, just as in physics, any action is met with a corresponding counter-action.
The thing is, Montenegro (independent from Serbia since 2006) was considered one of Russia's major allies in the Balkans. There is a significant trade exchange between the two countries, and Russian tourists provide the bulk of the revenue of Montenegrin tourism. But now the country is on the track to European integration. In 2014 they joined the anti-Russian sanctions because of the Ukrainian intervention. And then they accelerated the NATO integration.
The Russians are not happy, to put it mildly. Their press has pointed out that never before, since NATO started expanding, has this process been met with such sharp and painful reactions from Moscow - not even when the Baltics joined. And that's true.
Russia had responded to Montenegro joining the sanctions by banning Montenegrin wines on the Russian markets. Now these measures are expected to be expanded, possibly including an end to the Russian charter flights to the tiny Balkan country.
Of course, none of that will dissuade the Montenegrin government. But it might have a problem. Support for European integration has been far from unanimous. In fact their society is pretty split on NATO as well. Their government didn't even consider it necessary to organize a referendum for the NATO membership, despite the negative response of a significant part of the public. A country that was bombed by US planes in the 90s when it was part of Yugoslavia, still has a very influential pro-Russian opposition.
Russia is not helping their case much with its firm stance, though. Last October the authorities in Podgorica accused the Russians of planning the assassination of the then prime minister Djukanovic. Putin denied everything, of course. But the tensions are only bound to increase from now on.