How will node runners decide? Does the software have to be updated? Does this mean a hard fork for every "cautious change"?
What is part of the resource growth can be actually attributed to spam? Sources?
If legal risks are an issue, pruning might be an option. Or may be only keeping the last block after the initial download and verification of the blockchain. May be there are more intelligent ways of clearing your node of unwanted data, instead of trying to determine what is spam or what is not.
This opens the door to censorship and is by definition an end to the permissionlessness of bitcoin.