Alevtina,
I am on the Excel Services team and have discussed this with our Visual Studio for Office Developement team.
From what I understand you can add additional buttons, but you cannot remove them and Microsoft has its reasons for this.
http://blogs.msdn.com/b/luisbeonservices/archive/2006/10/19/customizing-ewa-in-xlviewer-aspx.aspx
If you need a more in depth answer, you can open a case with Microsoft and work directly with our Visual Studio for Office Developement team to get a definitive answer.
