Mikrotik Router 2.8.28
I use the mikrotik box for PPPoE connections. I want to block all ip addresses except those I have setup for users. What kind of firewall rule or rules can I use to do this? I have a forward rule now that basically says: drop all 192.168/16 on all interfaces. How can I do this for all networks except those I have setup for users (user networks are 10.1, 10.2, etc.)? Can I just put in rules to allow my networks and then the last rule is drop 0.0.0.0/0? So: allow 10.1/16 all, allow 10.2/16 all, etc. then the last rule is drop 0.0/0 all?
Can I just put in rules to allow my networks and then the last rule is drop 0.0.0.0/0?
Yup 
Beware that this may not prevent unauthorized users from “discovering” your scopes and setting statics based on subnets if this is your only method of securing your network.
Yes you can but as jarosoup pointed out, there are ways for folks to figure out the valid subnets.