…To review: The Memo From Your Future Self (and its variants) occurs in Chrononauts, Early American Chrononauts, and the Back to the Future Card Game. They can be played out of turn to cancel another person’s play as it happens.
A: Basically, yes. In cases where you’d discard that card from play in order to use its power, you could target that card with the Memo, causing it to be discarded without effect. That’s pretty straightforward.
For Back To The Future (BttF) only, note that if a Time Car is being memo’d it’s that card which would go into the discard pile without effect, but any required fuel item would remain (Plutonium, Lightning Prediction, Overpowered Locomotive, or the card discarded to use #4 would go back into the players hand).
Most Artifacts in Chrononauts don’t have powers, but a few that do would be affected: promo card Carl Sagan’s Joint, for example. In this case you’d normally give away the Joint and gain an Artifact from another player. If that forced trade is memo’d, the Joint would go in the trash (you were going to lose it to the other player anyhow) and the other player’s Artifact would remain with them. (I guess there weren’t a few puffs left after all…)
More complicated are Items/Artifacts/Gadgets whose power is passive. These include the (BttF) Gray’s Sports Almanac, Mr. Fusion, The Jade Statue of Tirade, any and all Gadgets. In these cases, you cannot stop the usage of their powers. If their power allows or involves the play of another card, THAT card may be targeted for cancellation via memo, but those cards already in play are not themselves affected by memos.
For example, when attempting to “feed” Mr. Fusion a card from your the table to gain two cards in your hand, it’s that card which would be targeted by the memo, and discarded with no corresponding benefit, and Mr. Fusion would remain on the table.
When “using” the Jade Statue of Tirade to get the extra bonus for Sell and Artifact or Perform A “Miracle”, it’s those Action cards which would be targeted and discarded, and the Jade Statue would simply remain in play, unsold, or unused as a “Miracle”.
The only time these cards with passive powers could be targeted by a Memo is when they themselves are actually being played to the table, in which case, instead of going into play, they’d go into the discard pile.