If you have removed the crank bolt, yet the cranks are still stuck on your bike, there may be a female thread on the crank in the area you have removed the crank from (see realistically clean photo below). If that’s the case, you’ll need one of these bad boys. 

The end of this tool has a removable head allowing the tool to be compatible with any type of crank. Thread the tool into the crank and then turn the chrome (shiny) part of the tool using a 15mm spanner.