Hide Ribbon button on form using javascript in crm 2015

var deativebtn=top.document.getElementById("button id");
 deativebtn.style.display="none";

Example:
var deativebtn=top.document.getElementById("cms_applicant|NoRelationship|Form|Mscrm.Form.cms_applicant.Deactivate");
deativebtn.style.display="none";

Hide Subgrid Create button from form using javascript in crm 2015

  document.getElementById("subgridname_contextualButtonsContainer").disabled = true;
document.getElementById("subgridname_contextualButtonsContainer").style.display = "none";
document.getElementById("subgridname_contextualButtonsContainer").style.visibility = "hidden";