Here is an excerpt from one of the popular Gnutella clients.
It is a part of the agreement the 'user' undertakes as a part of agreeing to accepting the terms of use as stated by those 'responsible' for creating and making available the software.
(I have removed the client's name and replaced it with 'xxxx'.)

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Blah, blah, blah and blah... The user...

... respects the rights of copyright owners to control commercial uses of their material. The Software enables users to connect to, download files from and share files over public and private Gnutella networks. By using the Software you acknowledge that you are responsible for complying with all federal and state laws applicable to the content you download and share using the Software, including copyright laws. Unauthorized copying, distribution, modification, public display, or public performance of copyrighted works may be an infringement of the copyright holders' rights in certain circumstances. You acknowledge that any liability for such infringement lies solely with you. You agree to indemnify and hold xxxx, its parents, subsidiaries, affiliates, successors, officers, directors and employees, harmless from any claim, demand, or damage, including reasonable attorneys' fees, asserted by any third party due to or arising out of your use of the Software.

By using the Software you acknowledge that you are aware that some files available on the public or private Gnutella networks accessible through the Software may have been created or distributed without the authorization of the copyright owner or as authorized by law. As a condition to the license granted under this Agreement, you agree that you will not use the Software to infringe the copyrights or other intellectual property rights of others in any way. xxxx reserves the right to terminate the license granted under this Agreement in appropriate circumstances upon any single infringement of the rights of others in conjunction with use of the Software or if xxxx believes the your conduct is harmful to the interests of xxxx, its affiliates, or other users; or for any other reason in xxxx's sole discretion, with or without cause.

As a condition to your use of the Software, you agree that you will not use the Software (i) to invade the privacy of, obtain the identity of, or obtain any personal information about (including but not limited to IP addresses of) any other user of the Software, or to modify, erase or damage any information contained on the computer of any other user of the Software; (ii) to infringe the intellectual property rights of others in any way (iii) to "stalk" or otherwise harass others; (iv) to collect or distribute child pornography or other obscene or illegal material; or (v) promote or provide instructional information about illegal activities, promote physical harm or injury against any group or individual, or promote any act of cruelty to animals or (vi) to engage in any other illegal activities such as treason or terrorism (this may include, but is not limited to, providing instructions on how to assemble bombs, grenades and other weapons).

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A Gnutella Network client can be protected by insisting that such a proviso be accepted by the 'user' prior to commiting to the installation of the 'software'. If the 'user' does not accept the terms, the 'software' does not install. Standard practice.

Any 'central server' based p2p client cannot be protected by the above or similar.

Yes ?