To determine the least number of bags you should buy so that you do not have any cashews left over, we need to find the common denominator between the $\frac{2}{3}$-pound bags and the 1-pound bags.
Since the common denominator of 3 and 1 is 3, we can say that the $\frac{2}{3}$-pound bags are equivalent to $\frac{2}{3} \times \frac{3}{3} = \frac{6}{9}$-pound bags.
Now, we need to find the least number of bags such that the total weight is a multiple of 9.
Let's consider some possibilities:
1. If you buy 3 bags, you would have a total weight of $3 \times \frac{6}{9} = 2$ pounds. This is not a multiple of 9, so there would be cashews left over.
2. If you buy 6 bags, you would have a total weight of $6 \times \frac{6}{9} = 4$ pounds. This is also not a multiple of 9, so there would still be cashews left over.
3. If you buy 9 bags, you would have a total weight of $9 \times \frac{6}{9} = 6$ pounds. This is a multiple of 9, so there would be no cashews left over.
Therefore, the least number of bags you should buy so that you do not have any cashews left over is 9 bags.