RichardHenry’s answer is the right one. No matter what you are publishing, you must know your audience, or the audience you wish to attract, and remain consistent with that vision. Your policy is directly related to your commitment to your audience.
You may not have to spell out “No racism,” etc. Depending on the type of publication, you may only have to say “We publish only material deemed by the editors to be of sufficient interest to our audience.” But you know what your yardstick is, and if a submission, whether paid or not, fails to measure up according to your yardstick, you decline to publish it.
The right of free speech does not obligate every publication to give space to your words. As long as I do nothing to prevent you from expressing or publishing your views, I am not interfering with your right to free speech. I do not owe it to you to publish them in my magazine.
Likewise, selectivity and appropriate discrimination are not censorship, nor is declining to publish something censorship. Every editor has the right and the obligation to serve his or her readership by exercising discretion in what is published. Censorship is the systematic suppression of content on moral or ideological grounds and does not describe an editor’s judicious application of selectivity to content or exercise of the right to edit material according to policy.