Barricade's already have a mob id: 1905 & 1906 i believe. Just make an NPC that will spawn the barricade 1 cell in front of them after they click it, and only let it work once per character, until it's dead.
mapname,x,y,z script Barricade Test 123,{
if(b_test){end;}
getmapxy(.@m$,.@x,.@y,0)
set b_test,1;
monster .@m$,.@x,(.@y+1),"Barricade Dummy",1906,1,"Barricade Test::OnDummyKill";
attachnpctimer;
initnpctimer;
end;
OnTimer60000:
set @min,@min+1;
if(@min==60){set @hour,@hour+1; set @min,0;}
setnpctimer 0;
end;
OnDummyKill:
set b_test,0;
stopnpctimer;
set @sec,(getnpctimer(0)/1000);
detachnpctimer;
dispbottom "You broke the barricade in: "+@hour+" hours : "+@min+" minutes : "+@sec+" seconds.";
@hour = @min = @sec = 0;
end;
}