@Laura2592 I think @Anonymous has a great point about how difficult it would be to regulate someone's cooking skills if you are allowing access to the kitchen.
Here are a couple of possible solutions that we have tried that have been successful for us so far. First, when we set up our listing, we decided not to have a cleaning fee, on the recommendation of a trusted fellow host, who said that her experience with cleaning fees was that it was basically a license for people to be slobs. Instead, we went with no cleaning fee, and we now emphasise strongly, in our manual, our house rules (posted in plain sight, with this one listed first) and in all of our correspondence to the guests, that we expect them to clean the place thoroughly before they leave. The day before they check out, we send them a short email--always on the Airbnb site--with checkout instructions and a friendly reminder to leave the place clean. To date, only one guest has left the place messy, but it wasn't the kitchen or barbecue area. Rather, they had infant kids who made a mess everywhere (story for another post), and they didn't bother cleaning up after them. But otherwise, this method has worked out well for us--so far, at least.
You also didn't mention whether or not your property is located near restaurants. If it isn't, then it is natural that guests will often choose to cook there. If it is, however, then what you can do is what we've done, which is to provide a list of the nearest restaurants, cafes and pubs in our house manual, as well as places that prepare great takeaway--and we offer an extended list of restaurants in the wider area, at check-in, to anyone who requests it in advance. (We always ask them first if there's anything they would like to know about, such as restaurants and tourist sites.) Not only do they always seem to appreciate the effort we've made here, but their decision to eat out frequently and/or to buy food from restaurants or shops and bring it home with them ultimately means they end up spending less time cooking--and that makes things easier for clean-up.
Anyway, these are some ways we've used to both assure our place is left clean and to minimise the desire for guests to use the kitchen and barbecue area. It can also help you come across as helpful and supportive rather than critical and judgmental (which is always a fine line you walk when you start telling people how they are supposed to cook).
We hope that you find some of these suggestions useful. Good luck!