Depends what you actually want...
If what you want is actually accounting for those users rather than obtaining "But you were warned in advance, now decision is already stamped" reasoning, then i have a suggestion: add into the library the $IfDef that would explicitly break the compilation in D2010 (unless some another term is defined, providing for you to still compile it for a while)
Something like
{$IfNDef DelphiXE_up}
{$IfNDef S4D_build_bot}
{$Message FATAL 'Delphi 2010 support may be phased out! Please immediatly report at
http://groups.....' }
{$EndIf]
{$EndIf}
Few would browse the forum until they do have a problem bothering them.
For the same reason it should be error or fatal, not mere WARN.