How to turn off "Allow Automatic Work Part Change".
Solution
Allow Automatic Work Part Change option can be toggled off in the Top Border bar next to the assembly filter. Allow Automatic Work Part Change is on by default and can be turned off in Customer Defaults under 'Modeling-->General-->General (tab)'. The option is located under the Grid Lines group at the very bottom of the Customer Defaults dialog.
Notes and References
Hardware/Software Configuration
Platform: all
OS: n/a
OS Version: n/a
Product: NX
Application: GATEWAY
Version: V1847
Function: CUSTOMER_DEFS
Ref: 001-9365340