Just from a first principles point of view, that has to be the easiest thing to control. If you put in priorities, intentional releases for bio-control would be the easiest, because that often requires approval by the organizations involved.
The live trade where people are intentionally trying to bring them in would be the next step, and it's also a very easy thing to regulate.
Natural means, where they're actually swimming in through the Chicago sanitary and shipping canal, would be harder, and it gets harder.
You could make a list. I remember receiving in my first position with exotic species a list of potential vectors in terms of zebra mussels. There was a list of 25. That was rather daunting to see 25 possible means, but if you put them in order, you could see that there are probably a handful that were important for the secondary dispersal of zebra mussels.
Likewise, you can put these in order. I think Dr. Ricciardi is correct in identifying the live trade as a low-hanging fruit. It is a very easy one to stop very quickly.