Players really can not effectively block a system or station. You can always switch to solo where those players do not exist and then switch back. That said, people who stay open all the time for whatever reason (immersion for example) can be blocked, maybe not 100% but enough.
A warzone is just a point in space where a war is going on. When you enter it, you should choose a side (in the functions menu). Choose wisely, you can gain and lose reputation there. Be ready to die if you are not an experienced fighter, there can be a lot of opponents there. Better use a cheap ship while you are still learning. To learn fighting i would suggest nav beacons and extraction sites, scan for wanted ships and blow them up.