Any of these border styles will produce a "non user-resizable" form:
bsNone,
bsSingle,
bsDialog,
bsToolWindow. The differences between them are basically the presence or not of border/non-client area (
bsNone vs. the rest) and the width and font size of the default non-client area (for all but
bsNone).
From the help for TFormBorderStyle (somewhat edited):
type TFormBorderStyle = (
bsNone, {No border at all.}
bsSingle, {Standard border, the form cannot be resized.}
bsSizeable, {The form can be resized (standard).}
bsDialog, {The form is a dialog, cannot be resized.}
bsToolWindow, {Standard border, small caption, not resizeable.}
bsSizeToolWin {Small caption, form can be resized.}
);