Mmm. I don't think what you want is possible
As with many sites Nabble generates a forum page each time it is displayed to a visitor. They are not static pages created, just once.
As they are not static then turning off the "updated on" feature "temporarily" would simply mean that as soon as you turn it on again the "updated" message would be displayed next time the page was viewed.
You either have to turn the feature off permanently, (as it is with Blog and News applications) by disabling the feature or let it run.
The problem is that it works on some data element that is related to the node number for the post and there's no way of an Administrator altering that. It seems to be you have no choice but to adopt the approach I suggested or never display the "updated on" line again for any post.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.