Turbo Mode on the i5 750 isn't adjustable. But there are a few things you can do to control it. With the i5 750 Turbo Mode is controlled (basic explaination) by the number of Active Cores and the Thermal Load. With 1 or 2 cores actively working the i5 750 will Turbo it's Multiplier to 24 (133x24=3192), if the Temperatures are within the Thermal Limit. With 3 or 4 Active Cores the i5 750 will Turbo it's Multiplier to 21 (133x21=2793), if the Temperatures are within the Thermal Limit.
So you have to control the Multi-Tasking that your PC is doing. This can be hard to do for everyday use, but for SuperPi you can disable any service that does background work like Automatic Updates, for example.
For Thermal Control the Best Thing you can do is to improve the Cooling with a nice After-Market Heatsink. And depending upon the options in your Bios you can disable the CxE Function and Intel SpeedStep.
SpeedStep allows the system to dynamically adjust processor voltage and core frequency, which can result in decreased average power consumption and decreased average heat production.
Processors - Frequently asked questions
CxE Function is a way to reduce power consumption by turning off certain parts of the processor and reducing voltage to others depending upon 3 basic states.
Intel Software Network Blogs » (update) C-states, C-states and even more C-states