Here is a bit of the case law on overly vague regulation of expression.
   - Carl "not a lawyer" Kadie

[From _Doe v. University of Michigan_, 721 F. Supp. 852 (E.D. Mich. 1989)]

C. Vagueness

Doe also urges that the policy be struck down on the grounds that it
is impermissibly vague.  A statute is unconstitutionally vague when
"men of common intelligence must necessarily guess at its meaning."
Broadrick, supra 413 U.S.  at 607, 93 S.Ct. at 2913.  A statute must
give adequate warning of the conduct which is to be prohibited and
must set out explicit standards for those who apply it.  Id.  "No one
may be required at the peril of life, liberty or property to speculate
as to the meaning of penal statutes.  All are entitled to be informed
as to what the State commands or forbids."  Lanzetta v. New Jersey,
306 U.S. 451, 453, 59 S.Ct. 618, 619, 83 L.Ed. 888 (1939).  These
considerations apply with particular force where the challenged
statute acts to inhibit freedoms affirmatively protected by the
constitution.  Smith v. Goguen, 415 U.S. 566, 573, 94 S.Ct. 1242,
1247, 39 L.Ed.2d 605 (1974).  However, the chilling effect caused by
an overly vague statute must be both real and substantial, Young v.
American Mini-Theatres, 427 U.S. 50, 96 S.Ct. 2440, 49 L.Ed.2d 310
(1976), and a narrowing construction must be unavailable before a
court will set it *867 aside, Screws v. United States, 325 U.S. 91,
98, 65 S.Ct. 1031, 1033, 89 L.Ed. 1495 (1945).

[From: _UWM Post v. Board of Regents of University of Wisconsin_,
774 F.  Supp. 1163 (E.D. Wis. 1991)]

B. VAGUENESS

A statute is unconstitutionally vague when "men of common intelligence
must necessarily guess at its meaning."  Broadrick v. Oklahoma, 413
U.S. 603, 607 (1973).  A statute must give adequate warning of the
conduct which is to be prohibited and must set out explicit standards
for those who apply it.  Id.  These concerns apply with particular
force where the challenged statute affects First Amendment rights.
Village of Hoffmann Estates v. The Flipside, Hoffmann Estates, Inc.,
455 U.S. 489, 499 (1982).  Nonetheless, the chilling effect caused by
an overly broad statute must be real and substantial and a narrowing
construction must be unavailable before a court will set it aside.
See Young v. American Mini Theaters, 427 U.S. 50, 60 (1976).