Kamil,
Your client devices ultimately get to set their own routing tables, it is on your VPN client config that it is decided if they try to use the VPN for all outbound traffic. That is not to say that you can't set policies denying that traffic on the DFL, just that the client determines where that traffic is routed in the first place. On a related note, you may wish to reconsider your position on accepting their default route traffic, as compromised or malicious users are much easier to detect if you receive all of their data streams.
For either of your points (almost) no traffic moves through a DFL unless there are rules specifically permitting it. Simply make sure your IP rules only permit that traffic you want from the VPN, and you can even write a specific rule denying traffic if that helps you sleep better. Traffic arriving over a VPN absolutely must have a specific rule allowing it, and you have the privilege of writing those rules yourself. The FAQs and docs have some rules and policies which are more general than you are looking for, but the necessary changes should be intuitive, replace lannet with your restricted range for example.
I hope all this helps.