When
-<
--distance
---Alpha 1
---Someship
--5
-self-destruct
--Someship
Of course, it's not perfect. Larger ships have a prolonged deathroll and hp has to be drained which self-destruct does not do instantly. But it would give you the desired behavior.
If there are several ships that the player could ram, using an event that includes arguments would be even better. As for the deathroll thing, there are ways to shorten that. Just prior to self-destruct, have a sabbotage-subsystem instance, that sets the hull to 1%. IIRC, sabbotage-subsystem occurs immediately. The other way to get around prolonged deathrolls, is by kamakazi. Essentially, flag the ship you want to destroy without a prolonged deathroll with a Kamakazi flag, either in the initial status tab for the ship, or via sexp. The later is probably a better choice, should the ship in question need to have ai-attack orders prior to its destruction.