For disabling of the spell check option please set the "ShowSpellCheck" property of the editmenu in the file ~\CMSModules\Content\CMSDesk\Edit\EditMenu.aspx to "false". This disables the button in CMSDesk. If you want to hide this button on a User Contribution, you should be able to apply the same approach on the file ~\CMSModules\Content\Controls\UserContributions\EditForm.ascx.