You should write something like this
function CallDelFunc() {
ExecuteOrDelayUntilScriptLoaded(deleteUser, "sp.js");
}
function deleteUser() {
}
<input type="button" value="Delete" onclick="CallDelFunc();" />
or you can load the function directly by using SP.SOD.executefunc instead of ExecuteOrDelayUntilScriptLoaded inside the CallDelFunc function
SP.SOD.executeFunc("sp.js", "SP.ClientContext", deleteUser);