Since you don't have access to a CSS file, I only see two options.
1 - Embed a stylesheet at the top of the static block. While editing the static block in question, click the orange Show/Hide Editor
button, insert your CSS solution mentioned above, and click save. Clear cache, and refresh page.
Here is a sample:
<style>
@media only screen and (max-width: 767px) {
.myclass{display:none;}
}
</style>
2 - Add a style tag and desired CSS to the head section of every page using the Miscellaneous Scripts
section in the Magento Admin.
- While logged into the Magento Admin, navigate to
System > Configuration > General > Design
. Expand the HTML Head section and paste your CSS solution mentioned above (including the style tags). Save the configuration. Clear cache, and refresh page. The same code sample above applies.