First Download Ribbon WorkBench.
Go to your Dynamic 365 site and choose Solutions from Settings menu:
Import Ribbon Workbench solution and install it.
After installation in solution page, you find a RIBBON WORKBENCH button on top of Page just above where All Solutions text available.
Create new solution and add those entities in which you want to hide your (+) button from the SubGrid.
Choose Button from SUB GRID and click Customise Button
Here I am going to customize two buttons (Add New and Add Existing) so I'll make some customizations for both buttons.
For hiding Add New option:
First Rule:
Second Rule:
For hiding Add Existing Option:
Display rule that I used to hide:
Output:
