Fouls (3.C): It is the responsibility of all players to avoid contact in every way possible.
Avoid contact in every way reasonably possible, while still playing ultimate. Some contact is inevitable, but players have an affirmative obligation to make reasonable efforts to avoid contact.
Non-incidental contact resulting from taking such a position is a foul on the blocking player.
A player may not take a position that is unavoidable by a moving opponent when time, distance, and line of sight are considered. Non-incidental contact resulting from taking such a position is a foul on the blocking player.
If you are already in a position, you maintaining that position is not "taking a position."