There are many triggers in Oracle Forms, which fires when user presses an associated key. Like for example when user presses F1 key then KEY-HELP trigger fires and so on. So if you just want to disable any default trigger functionality in Oracle Forms, then write form level trigger for that particular event.
Suppose you want to disable the create duplicate record functionality in Oracle Forms, then create KEY-DUPREC at form level, which fires when user presses an associated key and just put Null command inside as shown in below example:
Null;
This will disable the create duplicate record functionality of this trigger. Similarly you can write this for KEY-HELP, KEY-NXTBLK etc. Which sometimes we don't need.