Why are you disconnecting the calipers? changing hoses? or replacing calipers?
You can stop fluid from leaking as mentioned above... Cling film under reservoir cap before tightening back on will pretty much stop gravity drainage.
You can also crimp the flexible brake hose using a clamp or mole-grips, or lock-pliers.
When you re-fit the caliper you will need to bleed the system...
Take a look in the how-to guide seciton for advice on brakes and brake bleeding.