If it's a small one, cut the flowers off when they fade. If it's a mature one, I don't think you can!

what is the tree?

There are various hormone sprays on the market in the USA to stop trees from bearing fruit, but I don't know if any are available in the UK. The one I'm most familiar with is ethephon (the chemical trade name). Note that most of these sprays will do funny things to the plants under the sprayed trees, and may not be good for the environment in general.

Yeah, I suspect this one is banned over hear!

I was thinking about BIG fruit trees like our apple tree - totally impossible to spray with anything at all - unless I had a helicopter. LOL.