You can easily prevent the outposts with no need of obtaining a pass from the baron.
This task activates after attaining among few outposts in Velen’s boundary (M2,9) in which Redanian military has placed barricades and no one without a proper document is permitted to undergo.
You can bribe the person who you talk to (10 implants ) or utilize the Axii sign (25 experience points) to learn how to acquire a pass.
Nevertheless, these activities aren’t mandatory. The record can be obtained in two ways. The first alternative is finishing the Fake Papers (M2,6) sidequest in which you buy the pass from a shady merchant.
Second alternative is reaching enough advancement in Family Matters main quest. Baron Philip Strenger provides Geralt a List of Safe Conduct after he sees about the whereabouts of his family.
When you get the pass or the letter, you must show the document to the guard and he’ll let you via (75 encounter points).
If you don’t have the pass and you don’t wish to wait until you get it, you can simply skip every barricade by, for instance, using the boat to swim to another side.