I think your looking for these
setcell "arena",135,135,165,165,cell_walkable,0;
setcell "arena",140,140,160,160,cell_walkable,1;
The 'flag' can be 0 or 1 (0:clear flag, 1:set flag).
idk how its set up exactly but you can set up a line of them just experiment a bit
Then this for your message:
*mapannounce "<map name>","<text>",<flag>{,<fontColor>{,<fontType>{,<fontSize>{,<fontAlign>{,<fontY>}}}}}};
and i think you want these too :
*hideonnpc "<NPC object name>";
*hideoffnpc "<NPC object name>";
NPC names are unique you get errors for duplicates
Id put it together for you but its simple enough for a novice