I need to set a VLAN on that clients can't see each other. Something like with 255.255.255.255 subnet mask from PPPoE ISP. Is there a way to manage it via DHCP server?
You cannot stop clients from connecting across a layer 2 VLAN by imposing layer 3 IP restrictions.
If you need to separate clients, you need to put them into different VLAN's
Bart...
Your best option is to use switches that supports port isolation (or client isolation if you use access points).