I don’t think you can force it not to break. It would require a “non-breaking space” which I don’t think is possible in current or older versions of EndNote.Â
Just to add, that you could put a unique character that you don’t use elsewhere in the citation template (Author,~Year), and if it really matters to you, you could replace that character with a non-breaking space after unlinking fields and doing a search and replace, in word just before submission. But why do you care?