Spark did never have an ignore feature but Openfire and supports privacy lists. I have anyhow no idea if these work also with Y! accounts and how they must be configured to do.
You may want to take a look at http://wiki.igniterealtime.org/display/SPARK/Privacy+Lists and press F12 within Spark to open the debug window to identify the messages (especially from=“yahoo-user” - no idea if this is a unique name like “toad@yahoo-gateway” or just “yahoo-gateway”) so you know what you want to block.
If it’'s “toad@yahoo-gateway” you may want to use the Ad-Hoc commands (Spark+F12) and send
<iq id="id1public" to="firstname.lastname@example.org/spark" type="set">
<item action="deny" order="1" type="jid" value="toad@yahoo-gateway">
<item action="allow" order="1000"></item>
<iq id="id2public" from="email@example.com/spark" type="set">
This should set “block” as your default privacy list and block this user even if you logout and login again.