There appears to be nothing built-in to the AsyncTask API for this (most similar APIs have cancellation support, but libgdx's doesn't appear to).
You can implement it yourself. A simple approach is to use an appropriate thread synchronization primitive to control or act as signal to your asynchronous task that it should terminate stop working and early-out.
Properly guarded or marked up, a simple Boolean can suffice. Typically this means you will need to either check if the Boolean has been set to signal cancellation at the top of your tasks main loop and/or periodically throughout the execution of your task.
You can wrap this behavior up in your own utility class to avoid having to repeat it for every async operation you want to build, but even if you remove that boilerplate work you'll still have the issue where the cancellation is not immediate, it only happens at the level of granularity you perform the cancellation check. This is actually bad (because it means you could have a long wait between requesting a cancel and the actual cancel happening) and good (because it means you can make sure it's not possible to abort the async task while it's in the middle of transitioning some data between two valid states, and that way you ensure class invariants remain in place).
