My company has a similar policy. The logic behind the policy is that these kinds of spreadsheets/macros have no audit trail, or rigorous tests for viability/accuracy/safety. I've made dozens of macros in our test environment. I've made a few to rapidly fix a problem in production where the normal process would have taken too long, but that's the exception.
If they have a "No Excel in production" rule, you're probably not going to change their mind. You could point out some cases where Excel is used for some purposes, and they'd probably redefine it as "No Excel macros in production."
Pretty much your best strategy is to just ask "what are my alternatives?" What software can I use to achieve the same purpose? If using that kind of forecasting tool is part of your job description, they should have something. You could possibly ask how to get your spreadsheet approved, explain what it does and why it works, why it's safe, why you need it, but I doubt that would change their minds.
And don't be hard on them if you can't use it anymore. Financial institutions have a multitude of laws governing them, and a lot of oversight, and there are massive penalties if customer data is used improperly. So they tend to err on the side of caution.
Good luck.