It's impossible, from my limited understanding, to separate 'spam transactions' from 'spam data'.

A government could spam the chain by including transactions that literally move the same sats over and over... to the same addresses over and over.

Address A -> Address A -> Address A -> Address A... Is a 'spam' transaction.

Is it spam? Yes. Do you 'store' it on your node? Yes. It's data, that your node stores unfortunately.

However Blocks are limited to 4 mb.

The way to 'stop' or 'mitigate' the spam? Outbid it.