In recent version, The following works fine:
$('.selector').datetimepicker({
maxDate: new Date()
});
maxDate accepts a Date object as parameter.
The following found in documentation:
Multiple types supported:
Date: A date object containing the minimum date.
Number: A number of days from today. For example 2 represents two days from today and -1 represents yesterday.
String: A string in the format defined by the dateFormat option, or a relative date. Relative dates must contain value and period pairs; valid periods are "y" for years, "m" for months, "w" for weeks, and "d" for days. For example, "+1m +7d" represents one month and seven days from today.