Terrorist Isis have only been able to make gains in North Syria is due to Turkish support and supply.
That is correct. Considering that more than 30% of their fighters are neither Syrian, nor Iraqi, the ISIS is heavily dependent upon the Turkish-ISIS border to transport new recruits. If the border is closed, then new recruits will be unable to travel to Raqqa, and the ISIS will not be able to replace its fighters who lose their lives in regime and Kurd offensives.
Yes, and it´s important to close up general supplies, especially medical supplies. About the best morale-breaker is to have the wounded rotting away to contend with. War should be ruthless, brutal and as short as possible. This saves lives and suffering. It´s the opposite of the U.S. endless wars.