You can use Mastodon's "Filters" feature to automatically hide or block posts containing specific words, phrases or hashtags.

To access the feature:

1. Log in through your server's website
2. Go to ⚙️ Preferences
3. Click on Filters (on mobile this is in the ☰ menu, on desktop this is on the left of the screen)
4. Click on the Add New Filter button
5. Choose your filter's settings, then click Save New Filter

For more details on filter settings, see this guide:

➡️ https://fedi.tips/filtering-your-timeline-to-hide-posts-on-mastodon