In theory yes, in reality no, short of sinking their entire merchant marine.
That's simply not true even as of the previous version of the game before WtT came out.
Convoy routes have an efficiency. If the route is hit by convoy raiders, and it loses ships, there is some lag time where it operates at less than 100% efficiency. Assuming it doesn't get hit again in the near future, that efficiency climbs back up.
For trade, this can be abused by players, because you can reset the efficiency of a convoy route by cancelling and restarting a trade.
Supply cannot be abused in this manner; most of the time, players don't notice this, though, because it's easy to keep the routes open. When they close, or are raided badly enough, things get bad.
The sending and receiving ports must also have enough capacity to send the supplies.
Two examples that illustrate these mechanics in action:
In an MP game a few months ago, the Italians made a play for Turkey. The British army countered that move and took defensive positions across the straits before the Axis could get across. But Italian forces in Rhodes pushed into Anatolia to try and flank the Brits. The British army was stretched thin, but the RAF helped them by obliterating the port in Rhodes. Bomber Command bombed it and kept bombing it (to prevent repairs from resting it to functional status) until the Italians were beaten. Out of supply MTN divisions are no match for in supply Kiwis.
In another game, the Axis made a play for Portugal. They flipped Portugal before the war, and upon the war starting, Portugal joined the Axis. In order to secure the tungsten, Germany placed a substantial garrison force there. The good news was that there wasn't a lot Britain could do about it. The bad news was that even after France fell, Franco wouldn't join the Axis. Neutral powers will let trade go through their territory, so Germany could buy Portuguese tungsten without her convoys getting attacked. But neutral powers won't let supply through. So, there was a German convoy route going from southern France to Portugal. More than once, the Allies had that route down to 6% efficiency, and the German divisions in Portugal were starting to die on the vine. Attrition was becoming a real problem. Luckily, Franco finally joined the Axis, and the problem was solved. But for awhile, Germany was very worried that the forces in Portugal were going to get starved out and the Allies would walk right in.
