Well it's a Dutch company (Se Dig Online), so yo've theoretically got the same contractual rights as with a UK company (because the Netherlands are in the EU) but actually enforcing them might be a bit trickier.
Try an email to
[email protected] using your normal way of sending emails (i.e. avoiding any password-protected forms).
If you don't hear anything back, try phoning them. Phone calls to the Netherlands aren't expensive and the chances of the person answering the phone not speaking English are almost non-existent: 00 31 36 536 7931
Alternatively (and possibly as your FIRST way of resolving the matter) fill in your email address on the page referred to in your post:
http://www.simplesitebuilder.net/members/wwforgot.php
(ensuring that you use the same address that you first registered with), and click 'Send'. The company will then send you an email to tell you what your password is. Once you've got that you can use the login button, from the same link, to access your account and look for an option to terminate it.