League-of-legends – Minion wave clearing

There is a hotkey H which is the equivalent of toggling "Stop", which stops your current action. I'm not sure whether this will do what you need in stopping the auto-attack, but it's worth a try. I don't see any other options that could remotely do what you desire, other than getting stunned.

Continually pacing back and forth within range is a preferable course of action once you learn how to do it without getting exhausted. This actually has a few benefits besides just preventing you from attacking - it makes you a harder target, and it also makes it less detectable to be seen creeping closer and closer towards your opponent to make a pot shot. Alternatively, it makes your opponent uneasy since you can easily be in range to take said shot, which makes them less inclined to stay close.

The Spell Vamp portion of the LoL Wiki and my own experience show that

[Spell Vamp] works off of activated item effects, summoner spells, and abilities, regardless of the type of damage done.

Therefore, items that give proc effects to attacks like Bloodrazor, Wit's End, Malady, Wriggles etc, will NOT grant the user health through spell vamp.

On a side note, Ezreal's Mystic Shot (Q) applies on hit effects, including but not limited to, lifesteal.