Can I delete plugin files using FTP?

In almost all cases you can simply delete the plugin and it will be switched off automatically, and in fact that is advice I’ve given many times to help reel in broken sites. The same is true for themes.

Some plugins are actually more complicated than that and require additional steps. Notably, the caching plugins tend to fall into that category as some add Drop-Ins or create rewrite rules in .htaccess. Look at the docs for the plugin. If there are special unistall instructions you will need to address those. Otherwise, simply deleting it should be fine.

The chance of actually damaging data is pretty small in either case, though data can become inaccessible– or not easily accessible anyway.