That's because Ditto only presents content from it's own relay. It can and does filter on the backend using Nostrify, which can support NIP-13 PoW, but needs WoT implemented still.

https://nostrify.dev/

Otherwise it's up to reporting spam and hoping the operator sets up or improves the policy set.

Some examples: 
https://gitlab.com/soapbox-pub/gleasonator-policy

https://git.sleepingcro.ws/SleepingCrows/ditto-policy
(my personal attempt at combating replyguy, and pruning RSS, & porn bots)