[html] How do I disable the resizable property of a textarea?

You simply use: resize: none; in your CSS.

The resize property specifies whether or not an element is resizable by the user.

Note: The resize property applies to elements whose computed overflow value is something other than "visible".

Also resize not supported in Internet Explorer at the moment.

Here are different properties for resize:

No Resize:

textarea {
  resize: none;
}

Resize both ways (vertically & horizontally):

textarea {
  resize: both;
}

Resize vertically:

textarea {
  resize: vertical;
}

Resize horizontally:

textarea {
  resize: horizontal;
}

Also if you have width and height in your CSS or HTML, it will prevent your textarea be resized, with a broader browsers support.