new g_iClanIDs[MAXPLAYERS+1];

public OnClientPutInServer(client)
{
     // Store their current clanid.
     decl String:sClanID[32];
     GetClientInfo(client, "cl_clanid", sClanID, sizeof(sClanID));
     g_iClanIDs[client] = StringToInt(sClanID);
      
     // Change their clan tag.
     SetClientClanTag(client, "SM");
}

public OnClientSettingsChanged(client)
{
     if (IsClientInGame(client))
     {
         // Check if they changed their clanid.
         decl String:sClanID[32];
         GetClientInfo(client, "cl_clanid", sClanID, sizeof(sClanID));
          
         if (g_iClanIDs[client] != StringToInt(sClanID))
         {
             // Save their new clanid for checking again later.
             g_iClanIDs[client] = StringToInt(sClanID);
              
             // Change their clan tag back.
             SetClientClanTag(client, "SM");
         }
     }
}