How do I enforce a policy to block access to specific regions without updating all of my policies?

Service Control Policies do exactly what you've asked for. You can block regions, but beware some services are global so need to be whitelisted. For example IAM, WAF, Route53, CloudFront, some parts of S3 need to be whitelisted to run outside the permitted regions.

The AWS Service Control Policy Example page has this as the first example as it's the most common use case for SCPs. This policy denies regions outside the two listed, it's easy to change

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "DenyAllOutsideEU",
            "Effect": "Deny",
            "NotAction": [
                "a4b:*",
                "acm:*",
                "aws-marketplace-management:*",
                "aws-marketplace:*",
                "aws-portal:*",
                "awsbillingconsole:*",
                "budgets:*",
                "ce:*",
                "chime:*",
                "cloudfront:*",
                "config:*",
                "cur:*",
                "directconnect:*",
                "ec2:DescribeRegions",
                "ec2:DescribeTransitGateways",
                "ec2:DescribeVpnGateways",
                "fms:*",
                "globalaccelerator:*",
                "health:*",
                "iam:*",
                "importexport:*",
                "kms:*",
                "mobileanalytics:*",
                "networkmanager:*",
                "organizations:*",
                "pricing:*",
                "route53:*",
                "route53domains:*",
                "s3:GetAccountPublic*",
                "s3:ListAllMyBuckets",
                "s3:PutAccountPublic*",
                "shield:*",
                "sts:*",
                "support:*",
                "trustedadvisor:*",
                "waf-regional:*",
                "waf:*",
                "wafv2:*",
                "wellarchitected:*"
            ],
            "Resource": "*",
            "Condition": {
                "StringNotEquals": {
                    "aws:RequestedRegion": [
                        "eu-central-1",
                        "eu-west-1"
                    ]
                },
                "ArnNotLike": {
                    "aws:PrincipalARN": [
                        "arn:aws:iam::*:role/Role1AllowedToBypassThisSCP",
                        "arn:aws:iam::*:role/Role2AllowedToBypassThisSCP"
                    ]
                }
            }
        }
    ]
}