Perform the following steps in order to set ‘Default route table association’ and ‘Default route table propagation’ to disable:
1. Sign in to the Amazon VPC console at https://console.aws.amazon.com/vpc/
2. Choose Transit Gateways
3. Choose relevant gateway and click Actions -> Modify.
4. Uncheck ‘Default route table association’ and ‘Default route table propagation’.
5. Update route table with the necessary routes.

Via CLI:
aws ec2 modify-transit-gateway –transit-gateway-id –options DefaultRouteTableAssociation=disable,DefaultRouteTablePropagation=disable

From TF:
resource aws_ec2_transit_gateway” “example” {
– default_route_table_association = “enable”
– default_route_table_propagation = “enable”
+ default_route_table_association = “disable”
+ default_route_table_propagation = “disable”
}

References :
https://docs.aws.amazon.com/vpc/latest/tgw/tgw-getting-started.html
CLI: https://awscli.amazonaws.com/v2/documentation/api/latest/reference/ec2/modify-transit-gateway.html