You can hide the add to cart button. In General > Maximum Amount limit options > Hide add to cart button. You can choose to hide it on shop pages or single product pages.
To improve the user experience, do use the Messages feature to alert your customers why they are not being allowed to checkout. In the Messages tab of the plugin’s settings, you can set various messages.
We would like to let you know that, we have implemented the feature you wanted “Disable checkout button” in that plugin. You can find the settings here: WooCommerce > Settings > Order Min/Max Amount > General Options > Cart Page