To disable the resizable property of a textarea, you can use the CSS `resize` property and set it to `none`. Here's an example:

textarea {
 resize: none;
}

In this example, we're targeting all `textarea` elements on the page and setting the `resize` property to `none`, which will disable the ability to resize the textarea with the mouse.

Note that this may not work in all browsers, as the resizable property is ultimately up to the browser to implement. Some browsers may ignore the `resize: none` property, while others may provide alternative ways to resize the textarea.