I hate to be the bearer of bad news, but there's only one way, and that's to dig up the bulbs, carefully, preferably without leaving any behind (including the tiny offset ones) and bin them. Inevitably there will be some left behind, so every spring and autumn, I'm afraid its necessary to have a close look round the garden and immediately remove by digging out any new growth you find.

as already said digging them out is the best solution. how ever if you don't mind using glyphosate weedkiller then spray the foliage as they emerge and again on any leaves as they start to die back later in the year.

Have a look at this site, you may decide you want to keep a patch of it somewhere in the garden.
www.eatweeds.co.uk/allium-triquetrum

Even with strong efforts to eradicate it, there'll always be some somewhere.... I've been 'eradicating' this plant in a client's garden for 20+ years, and still patches appear, though significantly reduced now.

Numbersfarn thank you! What a wonderful site. The video alone was amazing. Have joined up right away