How we could decline a long-term customer’s request?

if it's really necessary to decline (which is not ideal especially if it's regular request and always being granted), you can apologize and inform that there's been some changes in the company's policies that needs to be implemented strictly. 

By not creating promises for the request done by the customer. By informing about the can do's & the other alternatives. The policies & procedures of the company can be explained as well. If all those facts can support the declining of the customer request, you may go ahead & decline the request in a polite, professional & a respectful manner.