Hi MerlijnH,
First of all, we know about the unfortunate issue with SB being available when overuse is enabled even if there's 0 (zero) SB websites in a Service Plan / on a subscription. We are going to fix this issue (hopefully in 10.4) in the following way: a resource that's completely not available will not be made available if overuse is enabled.
Second, I'm very curious about some of your statements and I'd like to ask you some questions, if you don't mind:
Q1: Why disabling overuse is unfriendly to your customers?
Q2: Why the option to overuse only disk space and traffic isn't sufficient in your case? To provide some background for this question: we've found that many people have complained about overuse policies in 9.x, so we've added a new option in 10.x. Now you can enable overuse for disk space and traffic only, which is what most hosters want, as far as we know.
Q3: What do you want customers to be able to overuse, except disk space and traffic? Maybe you have a different business model, so I'm interested in why you have to use "full overuse" instead of "disk space and traffic overuse".
Q4: Why exactly you do not use the Service Plans feature? Maybe you can share some business case examples that aren't covered by the Service Plans feature?
I would really appreciate it if you PMed me with answers to these questions -- I hope this can help us resolve your issues and provide us with better understanding of our user base, which in turn can lead to improving our products. Thank you!