You will find on Thoughts.com how to deactivate your account, however this option cannot be found in the process described. We’ll not guess for any reason. And we didn’t even mention account deletion. It is also the highest ranked question on their help forum: http://support.thoughts.com/forums/165532-thoughts-help-forum
So, you’ll probably get stuck here. Try the basic actions such as anonymising data (see below) and contacting them (see link above, then on your right hand side ‘contact support’).