Many accessories are designed to attach to a bicycle's handlebar. Those devices are vulnerable to theft if you leave them on the bike when you park it. But taking off all of your accessories at every stop could be a real hassle.
One alternative is to use a readily detachable accessory bar, like the one shown here.

I attached my accessories to a spare mountain bike handlebar, then strapped that handlebar to the bar ends of my bicycle.
I secured the bar to each bar end using a Lockblock, made by Twofish Unlimited. The product's package contains three of the Lockblocks, but only two are needed.

A pair of their Cyclopblock product might work also, but I have not tried that.
Even with the accessories relocated, it's still possible to use them while gripping the bicycle's own handlebar. Below you can see how I move a bell lever, press a horn's button, and press a light switch.


When I lock up my bike in public, I can undo a hook-and-loop (Velcro) strap on each Lockblock and just lift the accessory bar away.

I either carry the bar away in my hand or put it in my bag. As you can see, the bar does not quite fit in the bag, but it sticks out only a little.