Technically no. Nothing on the internet is safe. Anybody with the ability to copy/paste can take it but most won't, and then there are the content scrapers which are computers that just copy content and post it elsewhere. There are measures you can take though to alert you if someone does steal your content and steps you can take to getting it back.
There are ways to set up google alerts https://hubpages.com/technology/Google-Alerts to notify you if someone has reposted your content somewhere else. Just have it alert you when someone uses a line that is unique to your poem.
You can also use www.copyscape.com to check your hubs to see if they are posted anywhere else on the internet. You are allowed so many check for free per month then it makes you pay for credits but it is totally worth it. I even check other people's articles if I suspect they are copied.
If you find your article has been stolen you can file a DMCA complaint https://hubpageshelp.com/promotion/Learning-Center... and most of the time the other site will remove your content without a problem.
But don't let content thieves deter you from writing on the internet.