How to disable appearing of ToolTip text over ToolStripButton?

Try the following setting:

btn.AutoToolTip = false;
btn.ToolTipText = string.empty;

The note on the AutoToolTip property:

Set the AutoToolTip property to false to display custom ToolTipText on a ToolStripItem.

You can also disable all tooltips just go to toolstrip properties and then select False