patio weed killer Pathclear which has a residual effect to stop seed germinating. If you want to be eco friendly i understand that salt and water, 3 parts water with one part salt dissolved in it also works
Lawn weedkiller is more iffy, i have never seen it work that well because its a fine line between killing weeds and killing everything. Mostly if you check all the selective weedkillers, they have the same ingredients.
https://www.rhs.org.uk/advice/profile?pid=369
You can also spot treat with a glyphosate or diquat based spray but be careful as these will also kill grass. Frequent close mowing will also deal with many of the coarser weeds and encourage the grass to spread into bare patches.
Or you can take my approach and have a mixed wildflower lawn! (ie do nothing!)