For trade offers that won't cancel, according to https://scrap.tf/help/trade-offers: "This means that either the trade offer is in the process of sending, or the bot has suffered a failure. In which case, you might need to wait up to a half hour for the bot to restart. If you have been stuck for more than 30 minutes, then we suggest you to ask a supporter in the support chat to kick you out from the queue. If support happens to be closed, you can submit a ticket regarding it in the Help and Support forum (you have to be logged in to access it)."
The problem with this is that 99% of the time, nobody is in the Live support chat and the Help and Support forum guys usually take hours to respond. So it would be great if we could like press a button to kick ourselves out of the queue instead of having to get an admin to do it.