When you say a variable block, do want to vary a signal at source by using the signal builder block, or repeating sequence block? Or do you want to vary a signal based on other conditions in your model? This could be done using a switch in the signal routing library.
If you have Inline Parameters turned off (it's on the Optimization page of the Configuration Set), you can just open the block dialog and change the value. If you want to use a workspace variable, then you can change the value of the workspace variable and do an Update Diagram (^D) while the simulation is running.