antlr-options-auto-colon is a variable defined in `antlr-mode.el'.
Its value is

Non-nil, if `:' is inserted with a rule or subrule options section.
A `:' is only inserted if this value is non-nil, if a rule or subrule
option is inserted with M-x antlr-insert-option, if there was no rule or
subrule options section before, and if a `:' is not already present
after the section, ignoring whitespace, comments and the init action.

You can customize this variable.