Rules Analysis
Every player is entitled to occupy any position on the field not occupied by any opposing player, provided that they do not initiate contact in taking such a position.
If a player does initiate contact in taking such a position, this is a Blocking foul (17.8). All players have a responsibility to avoid initiating contact, even if that means that they must pull out of making a play on the disc.
However when the disc is in the air a player may not move in a manner solely to prevent an opponent from taking an unoccupied path to the disc.
What:
Player A, who is making a play on the disc, is allowed to slow down and to impede a opponent's movement to make a play on the disc. However Player A must not move in a way that the opponent could not reasonably avoid them -- this is a Blocking foul (17.8). Some incidental contact may occur in these circumstances but incidental contact is not a foul.
Extra:
If Player A is not making a play at the disc, but is instead allowing a teammate to make a play at the disc, Player A may not move to impede an opponent. However if Player A is stationary, or does not intend to impede, this is not a violation, even if their actions do in fact impede an opponent.
Note:
The key word in this rule is 'solely'. The intent of the player's movement can be partly motivated to prevent an opponent from taking an unoccupied path to the disc, so long as it is part of a general effort to make a play on the disc.
Every player is entitled to occupy any position on the field not occupied by any opposing player, provided that they do not initiate contact in taking such a position, and are not moving in a reckless or dangerously aggressive manner.
If a player does initiate contact in taking such a position, this is a Blocking foul (17.4).Moving in a 'reckless manner' could include running without looking where you are going for an extended period of time, or diving in a way that does not allow you to adjust to any legal changes of movement that an opponent might make.