Yes, this happened very early on with Bitcoin. Casinos where every wager was a transaction. Then some started actively just spamming. It led to the transacti

Yes, this happened very early on with Bitcoin. Casinos where every wager was a transaction. Then some started actively just spamming. It led to the transaction pricing mechanism. Every transaction includes a bid on how much you'll pay to have it included in the chain. Miners pick the highest bids from recently broadcast transactions. There's no requirement to do so but it's economically incentivized, as the miner receives the transaction fee. Any sustained attack becomes very expensive. And other users can still pay the asking price to get their transaction in the next few blocks. Bitcoin has in practice maxed out the great majority of blocks (about 1 MB per 10 minutes) for a while now, and so many cheap transactions just never get processed as they're always outbid.