I don't know any way to keep anything 100% safe on the Internet except not publish it there.  A small protection would be to put it on a website that has password protections so that entry is restricted.

This is something the artists here struggle with all the time. They create something and then find someone else is displaying it on their website and claiming ownership of it. It is often a long and tedious process to get a thief to cease and desist.

Did you hear about a college student who was recently exposed as stealing from another young woman's published work?  Her book was published, and copyrighted, before it was discovered.  Even getting published doesn't protect yourself from theft--there's just no 100% solution.  But because she was so clumsy in her plagarism, people spotted the identical phrases from one book to the other and the book was pulled.  Your handwritten pages, dated, could be some proof of your effort having predated that of the thief.  There is also a volume of work. If you have written poem after poem after poem, and your thief has plagarized only one, you have proof of activity that presumably your thief doesn't.  No sure thing.

But my advice stands. If you intend to be published, don't put your poem on the web.  At least if you publish in a book or anthology or as part of contest, you may have some support from the publishing company to help you.