Generally speaking, you cannot. However, if your blocks are based on tables, meaning you are not performing your own SELECT * FROM EMP, then if you set the form's "Interactive Mode" to Non-Blocking, a Cancel option will be displayed after the query has run for a brief period. Refer to the Builder online help for more information.
It is not possible to cancel your PL/SQL or procedures or triggers of your forms or db
but if your block is based on database property yes and you are querying on the block like using EXECUTE_TRIGGER then your can use
set the form's "Interactive Mode" to Non-Blocking
By doing this your forms can give you the control to use to cancel the database query.