You are able to stop your contributions whenever you wish. However, as a general rule you are unable to switch between cash (e.g., salary) and non-cash benefits (e.g., pension) whenever you like because the tax and NI contribution saving under a salary sacrifice arrangement will no longer apply.
If you do ever want to change the terms of the arrangement by amending the amount or restarting contributions this can be considered where a change of lifestyle significantly alters your financial situation. HMRC classifies a significant lifestyle change as things like marriage, divorce, a partner or spouse becoming pregnant, or a partner or spouse being made redundant. In addition, this may include a change to a key contract or assignment.