Hi
Check the customizing via following path:
1. SPRO->CRM->UI framework->Technical role definition-> Define navigation bar profile or execute CRMC_UI_NBLINKS
2. Select the navigation bar profile Z*(or corresponding one),
3. Double click on 'Define Generic Outbound Plug Mapping'. (There, the object BP_CONTACT with the Obj. Action D Create can not be found)
4. To disabled the new button. remove entry below
Object type: BP_CONTACT / BP_Account
Obj. Action: D create
Use target: checked
Target ID: MD-BPCP-CR
Highlighted Link ID: MD-CONP-SR
You should repeat these steps for the relevant Navigation Bar profiles
1792195 - New button disabled on Contact assignment block in Account Overview
Thanks,
Vaibhav Shah