too answer the question, yes you can clamp off the rubber lines, with hose clamps. they are kind of specialised item, not in all automotive stores. a well stocked hardware store might be a good sorce, as would a internet search. I have some hose clamps that are realy cool, great for feul hose, they are all metal, and via a screw pinch down on the hose via a flat bar with rounded edges aginst another bar. Dont remeber where I got hem however.
You may also buy some small C-clamps, or the tired and true method of shoving teh correct diameter drill bit into the hose end, (quickly!!!) and clamping it down.
vice grips, if carefully aplied (nose across the hose )can also work nice. but note heavey big vice grips or large C-clmaps can weigh so much you have to over tighten them (hence harming the hose) just to prevent gravity from pulling them off (which would give a massive fuel leak!!)
while yoru at the gas tank, now is the time to do any maintanace on the fresh air box , lube the fan, un-clog the drains holes, also any wiper lube should be done now as easy access!
If you have an early car laking the cowl grill screen, get one and install it to keep debries out of the air box, in addtion to the later years screen, consider adding an aluminum window bug screen on top of the factory platsic screen, the factory screen is still too course, and all sort of stuff can still get in.
good luck
when I work on gas tanks, I get real nervvous, nothing like a smoke to calm me nerves as I work.

PS although you can drain the system as suggested, you will still be advised to clamp the lines as fumes may still escape and debries may enter any uncapped line. Draining the system does not negate the need to clamp the lines closed