There's
nothing that will prevent someone from downloading your song.
Anyone can record the sound coming from their speakers. This is beyond your control. I can set my inputs to Mixer-In and anything coming from my speakers will be recorded.
But most computer users won't know how. And if people like your music enough to record it then they will probably just buy it.
I would also link to SoundClick because it gives people the option of buying your music. Putting that into your website will take some work. Unless you already have that built into your site.
Edit: Also, the MP3 file can be downloaded from any site (even SoundClick) that supports streaming. This is harder than recording what comes out of the speakers, but it's still definitely possible.