If a product in your online store has only one variant, you may be looking to hide it since there aren't any other options. There are a couple of methods to consider to hide that single variant.
1. Remove the variant information from your product
A simple solution. When setting up your products, you may be adding variants whether you actually need to or not. If you have a unique item that will never come in another version, there wouldn't be an option that your customer would need to choose on that Product Page. So, adding this information to the Product's admin page wouldn't be required.
You can also consider naming your product with the specifics. For example: "Red wool hand-knit scarf." Or, include it in the product description ("This lovely hand-knit scarf is made with 100% merino wool in bright red...") This information can be incorporated here instead of adding a Color with one variant (Red) and a Material with one variant (Wool).
2. Hiding the drop-down/swatch with custom code
If you want to add or keep the variant information for your Product in the Product's admin page but would like the variant to be hidden on your storefront, then you will need to modify the theme's code.
For code customizations, we recommend seeking the assistance from a developer or Shopify Expert who would be able to implement the changes. We have a list of recommend Experts here.