Azure Cognitive Search - how to prevent searching for plural form also returning singular matches

Plural and singular forms are likely both matching because the field is configured with the default language analyzer, which performs stemming of terms. If you're looking for an exact match, you can use the 'eq' operator in a filter. If you want a case-insensitive (but otherwise exact) match, you can try normalizers (note that this feature is in preview at the time of this writing.)

If you need matching behaviour that is somewhat more sophisticated than a case-insensitive match, you should look into custom analyzers. They allow you to customize the behaviour of tokenization, as well as selectively use (or not use) stemming and other lexical analysis techniques.