That is definitely unfortunate, unforeseen costs are never nice to find, especially if these charges have been around for quite some time now. From my understanding you have created a Cloud SQL instance for SQL Server on your project,
related to a public quickstart.
As long as the only resources utilized for your implementation were the Cloud SQL instances, then yes,
deleting the instances would prevent further charges from occurring on the Cloud SQL side. If you utilized any other resources such as,
App Engine or
VPC Networks, in order to utilize the Cloud SQL resources on your project, it is possible that there may be other existing charges.
In this case I would also recommend deleting these instances.
Although not much can be mentioned about SQL Server 2017 and how to manage costs for this product, as it is a third party product managed by Microsoft.
If you have any further inquiries about billing and any further doubts about why your account is being charged,
please contact free billing support, they will be the most efficient means of investigating these types of inquiries.
I hope this helps, and good luck!