SetRespawnMask(), unlike most flag verbs, sets the total flags instead of only adding flags. So there is no ClearRespawnMask(). GetRespawnMask() is used to retrieve the respawn mask of a player. Respawn masks must be set for each level that they're used in, because they will not be saved for the next level.
If all respawn points in a level are masked for a player, then he will spawn at the first walkplayer. These "spawn points" are the positions of walkplayers in the level, but not the players themselves.
The order of spawn points corresponds to the thing numbers of the walkplayers in the level. So the first spawn point in a level is the walkplayer with the lowest thing number. And the last is the walkplayer with the highest number.
If, for example, you wanted to make a player use only last spawn point in a level, and the level had 16 walkplayers, then you would use a mask of 0x7fff. Note here that each digit can mask up to 4 spawn points. So the three 'f' digits block 12 spawn points. The 0x7000 is the combination of 0x1000, 0x2000, and 0x4000. The last flag for that digit, 0x8000, is not present. The lack of that flag allows the player to use the last of the 16 walkplayers as a spawn point.