A quick search revealed this Non-retracting seat belts 2006 NC - Technical Area / Body, Interior & Styling - MX-5 Owners Club Forum (mx5oc.co.uk)
(The silicone spray sounds worth a try)
It is a well known issue on the NC so there may be other threads to look at. I suspect that there is little difference between NC and ND seat belts as they are virtually identical structure wise in this area.
