registered your trademark with the USPTO, and (2) registered your comic strips with the U.S. Copyright Office. Those procedures enable you to chase infringers, assuming you have the money for an attorney. If you're looking for additional means of chasing those who use your work without permission, you can require users to enter into an end user license agreement (sometimes referred to as an EULA). Under that arrangement you may attempt to further limit your client's rights and you may establish methods of resolving disputes (for example, requiring arbitration, not litigation). Finally, if someone posts your work without your permission, you have the takedown provisions of the DMCA at your disposal.
Fair use? As you know, fair use permits limited unauthorized uses of copyrighted materials for purposes of commentary or criticism. To the consternation of some in copyright law, a few courts have held that a license agreement can negate fair use rules. Whether fair use trumps a license agreement is ultimately up to the courts. We can't tell you "how much" fair use protects -- it's not a mechanical calculation -- so, you'll need to review cases with similar facts to determine your possible outcome. But we can tell you that if someone makes an unauthorized use of your material, and they're using it for the same educational purposes as you intended, and depriving you of revenue, it's probably not a fair use.