CSS

disable textarea resize

textarea {   /* for all text* area elements */
  resize: none;
}

#foo {                  /* for particular id */
  resize: none;
}

textarea resize off

<textarea class="myTextArea"></textarea>
<style>
  .myTextArea {
    resize: none;
  }
</style>

remove resize from textarea

textarea {
	resize: none;
}

asp.net textarea disable resize

<asp:TextBox ID="TextBox1" runat="server" TextMode = "MultiLine" style = "resize:none"></asp:TextBox>

make textarea not resizable

You can either apply this as an inline style property like so: 
<textarea style="resize: none;"></textarea>
or in between 
<style>...</style> element tags like so: 
textarea { resize: none; }