I was having this problem too, even after selecting “Don’t show any related products”.
It looks like the problem affects products created or last edited before the Custom Related Products plugin was installed.
This solution seems to work consistently:
1. Go to Dashboard -> Products
2. Select all (on the first page) by clicking the checkbox on the top left
3. Click on the Bulk Actions select box, select Edit and click Apply
5. Scroll down and click the Update button on the button right of the Bulk Edit Dialogue (you don’t have to actually change anything)
Oddly enough, for me this made the random related products dissapear on all products, including the ones that aren’t on the first page. You could also try going to each product individually and clicking Edit then Update if that doesn’t work
