That is very unusual, but we are happy to help: in general, if you follow the instructions here
you can add your custom HTML to a 'Text widget' (which allows HTML), and if you choose the simplest option [B] it should work fine - in any event, there should be no side effect on any other sidebar content anyway!
Please get in touch with us directly [via our contact page] if this problem persists.
All the best,
-CJ on behalf of the ClustrMaps Team
NOTE ALSO:: you can choose much nicer map styles on the Admin page (see 'Map styles' tab on the right). All users are eligible, although ClustrMaps+ users have a larger range of shadings, dot colours, labels and languages ... see some representative examples here: