I am late on in a game and don't need science any more (as I have researched everything). Is there any way to 'turn off' science and convert it into something else e.g. Gold, Culture or Production. For example is there any way I can sell my science buildings (such as University, Public School etc) to raise some cash?
I think the best way is to manually allocate citizens in your city to work for things that generate more gold / production / food / culture over science.
For example, if you leave your citizen allocation for the "governor" (the AI), it will surely prefer a "2 production / 5 science" hex to a "2 production / 1 gold" hex, even though the latter is better for you. Remember that for manual allocation you must first open the citizen allocation menu in the city screen, and also remember you can allocate citizens in buildings, for the buildings that allow it (called specialists).
You can also leave the allocation to the AI and only direct his effort by choosing something to focus on, via the radio buttons in the manual allocation menu. Those work better when you want something specific, though, not when you want to avoid something specific.
Finally, remember that even after you have researched everything, you can continually research the repeatable "future tech" technology for extra points. It gets harder and harder to complete it every time you repeat it, so you can never have too much science if you go this route.
By the way, if you have built any academies you can now override the hex containing them with some other improvement / great person structure, that can also help if you want to redirect your efforts elsewhere.
EDIT the newest version of civ5 allows you to sell buildings - which is perfect for this case! Just sell all your research labs, universities, libraries etc. - sell all buildings that only give you science. Since you can sell only one building per city per turn, I recommend you start by selling the buildings with the highest maintenance cost.
No, you can't turn off science.
Unlike Civilization 4, there is no economy that gets split into gold/science/culture/espionage. Instead, science is created directly as science.
So you're not directly losing anything be leaving your science on. Instead, you're actually gaining additional score each time you complete a future tech, so your victory will be all the greater!
The best you can do is what Oak suggested: switch your citizens to working on something else, either through focus or manually stopping them from working on science squares or as specialists in science buildings. Where your citizens work is about the only way you are losing other resources from producing science, so changing what they work is all you need to do.