I'll usually add $DEST := "@P" + $PAGEID on items to remove the pop-up, "The item cannot be used here." It seems to pop up regardless of whether you can trigger the script or not. By adding it like Bradin did, it'll prevent the pop-up when you can use the item, but allow it when you can't use it.
If you want to stop people from accidentally using a consumable when they're already at full health, you can add a second layer of logic to it. You also want to make sure their health doesn't go over the max health, making it only fill up what's missing if they have less than the full heal amount gone. It'd roughly work like this:
If variable is greater than 0 then
If current health is less than max health then
BEGIN
Set health to health + 10
Set variable to variable - 1
If health is greater than max health then
health := max health
Set destination to current page
END
