The seedling stage is the best time to spray, and you can generally kill it with a single application at that point. Once it is established, and has a deep root system, it will take repeated applications over the next few years to eliminate it. Most herbicides (ex. 2-4 D, dicamba, and glyphosate) need to be applied before flowering. You can find a list of effective herbicides and timing of application in this report from Weed Control in Natural Areas in the Western US. One other note, spraying during bloom will impact pollinators and other beneficial insects, and should be avoided. Malta starthistle reproduces only by seed, so you should remove the plants before they set seed through hand removal, mowing, or cultivation. For more on Malta starthistle, refer to https://www.nwcb.wa.gov/weeds/malta-starthistle