I have not had much chance to work on those switches, but I am fairly confident that the ARS (Automatic Route Selection) programming is pretty much the same as in the SX-200 and 200 ICP.
I would first go to the form that would have a name like ARS COR groups (I think it would be form 23)and make a COR group that includes ALL COR numbers 1-50. When this COR group is used, all devices will be restricted. COR stands for Class of Restriction.
Then I would go to the form that would be for ARS routes (form 24 I think) and create a new route. This route will have a valid trunk group, digit modification number (doesn't matter which one), and a COR group number. Use the COR group number you just created.
Then you would need to go into form 26 - Leading Digits.
I am assuming that your switch is set up to dial "9" for an outside line. You would chose the leading digit 9 entry and expand to see what following digits are defined.
I would create an entry that would look like "1900 + 0 digits to follow and program that these calls go out the route number that you just created.
The call will not proceed because of the COR setup.
I would consider setting up similar digit strings for "976" numbers as well using the same route you just created.
The only down side of this would be if the COS (Class of Service) of the device is programmed for Abbreviated Dialing to over-ride Toll control. Then someone could (if smart enough to realize it) program the number as a system speed dial. Just check the COSs used and make sure that this feature is disabled.
Good Luck.