I think there is little you can do about since this is Netscape's fault.
The onliest thing you could do is warn your users or completely remove the clicksmilies from your template.
Because of the little CSS support and some other glitches in Netscape 4 I'd recommend your users upgrading to Netscape 6. But of course it should always be a goal to make your website viewable with all Netscape and other non-MS browsers.
For the ones that don't can or want change browser, a warning is the best thing you could do (imho).
I'm using Mozilla, Netscape 6 opensource base and I have no problems typing e.g. my name -> ƒVƒ‚ƒ“-‚³‚ñ 8) <- and using smilies (
(inserted via click (ye I know I can't prove))