The best thing is to find a project that will cover the profile and protect it.
While it is not standard practice, I have used this at the top of a profile.
<div style="padding-bottom:5px;text-align:center;"><span style="font-size:200%;color:red;">While many genealogical sites have this person, there are no records on any site to prove their existence.</span><br />~~~~~~~~</div>
Just change what is between the two "spans"