If you know no one will mess with it, you can also use a velcro strap on your brake lever, regardless of brake type.
If the lever housing has nothing in the way, then you can pull the lever and hold it out of the way with a strap, then drill a hole in the lever housing for a pin, in a place that blocks the lever open all the way. (don't drill into the lever itself) Be sure you aren't compromising the housing's structure with the hole (it has to go all the way thru from top to bottom). If you use a "cotterless pin" with an attached metal lead or ring,
https://www.google.com/search?q=%22cotterless+pin%22++with+lead&tbm=isch
you won't have to worry about losing it or forgetting it, and can be left hanging from the lever area when not in use.
If you ahve disc brakes on at least one wheel, there are disc locks that will fit a number of types. Most are for motorcycles, but some fit bicycles.
https://www.google.com/search?q=brake+disc+locks&tbm=isch