public OnEntityCreated(entity, const String:classname[])  
{  
          if(StrContains(classname, "env_smokestack") != -1)  
          {  
              RemoveEdict(entity);  
          }  
}

#include <sourcemod>       

public OnPluginStart()       
{       
       HookEvent("round_start", round_start);       
}       

public round_start(Handle:event, const String:name[], bool:dontBroadcast)       
{       
    new String:class[50];
       for (new i=0; i<=GetMaxEntities(); i++) if (IsValidEntity(i) && GetEntityClassname(i, class, 50) && StrEqual(class, "env_smokestack")) RemoveEdict(i);
        
}