Roddles, I haven't got a solution to your problem (and pat's problem sometimes, and no doubt other people's problem too). I had a Samsung tablet myself for a couple of years, and didn't notice such a problem, but of course Samsung makes a multitude of different models, which might react slightly differently to touch screen actions.
Your letter selection will be treated as a word submission too, if you hold down your finger too long, because the letter will have moved away so that your finger is now on the green arrow. But if you try to give each letter a short sharp jab, your finger might bounce, leading you to do an inadvertent double-tap. Obviously you need a Goldilocks touch - not too slow and not too sharp, but just right.
I'll have a think about any adjustments to the mechanism that might make things a bit less "touchy", without interfering with the option of using a double-tap to complete the word.