Thanks for responses.
It will cost you money, and it will make the miners (or mining pools) very happy to collect all those transaction fees from you.
And you can spam transactions, but if you don't include fees, it will take a lot of time to proceed.
Alright I didnt think about the fees at all. What happens to transactions without / with super low fees? They simply don't get included if the one who mined the block doesn't want to right?
However, these non-blocked transactions must still show up somewhere, like be stored as "please insert me for free" somewhere? Is this still spam-able?
Unlikely. Unless the protocol changes, it can't grow by more than 1 megabyte per block.
If there is enough transactions to take up more than 1 MB then what? The block can't take them and transactions stack up?