I give a LOT of Ride On thumbs ups. I especially like to give them to people who are stopped or moving slowly up an incline, as encoragement. I’d like to be able to see (different coloured font?) riders I’ve already done so, so I don’t waste a group ride on when there is someone fairly nearby that I’ll miss while it times out. You could leverage the fact that it knows who I’ve already interacted with so that it doesn’t allow a second one in the same ride.
Zwift already only allows you to give one Ride On per ride to another rider, so you shouldn’t be able to send multiple. Also, if it makes any difference to you, because the ‘Ride On’ increases the number of Drops the receiving rider gets for a period of time, giving a Ride On to a stopped rider won’t do anything with regard to that, since there are no Drops earned while stopped, and 0 x anything is still zero. (Not to say that receiving the Ride On won’t provide some motivation, just that there is no Drops multiplier while not putting out any power.)