Noob GM looking for help on something

I am running my first 5e campaign on roll 20, We are in the Dungeon of the Mad mage 5e. I suspect one of my players may be exploring the dungeon when im not running. Is there some sort of toggle i can use to lock players out during the week then unlock it during the weekend when i run? 

You can move the ribbon back to the title page. Players cannot change the page (map) they are on. Also, if you are on a page with dynamic lighting, make sure it is set to "update on drop". That way a player cannot skim their token throughout the dungeon without releasing it and surreptitiously sneak a peak.

oh ok I didn't think of that thank you Kieth :)

Another thing you might want to do (if you aren't already) is put all the enemy tokens on the GM layer until you need them.  Drop them onto the token layer as the group becomes aware of them through sight, sound, etc.  Then, even if people peek ahead, there really isn't much to see.

I had a tiny suspicion that my players were dragging doing the odd bit of sneaking ahead actually DURING the game. I don't think they were properly 'trying to cheat' as such, but were perhaps just a little to eager in some places. They're all experienced on roll20, but none are expert DM's on here. - I just told them that I get 'alerted' in the chat window each time a player token is moved. It's entirely nonsense, but they don't know that :) - but it makes them think they'll get caught if they do move ahead! :) Both suggestions above are spot on too. EDIT: I love the suggestion below too! I may have to try this. I did once write on the floor of one room.... "If you're reading this, then it means that either the dynamic lighting feature isn't working at the moment (If so, my apologies).......... or you're sneaking ahead! Naughty!"

While my primary solution would be a social contract to NOT CHEAT coupled with a splash page for between sessions I do also like to mess with my players... A fun thing to do if you suspect this is to place a terrifying token in an area you'll cover in the next session... maybe an ancient red dragon or a dozen mind flayers, you'll soon know who is meta-gaming/cheating when they are suddenly reluctant to go in one particular direction ;)

That is the stuff of evil genius!

Or use Its a Trap and put in some nasty surprises. :)

Thank you all for the suggestions the game is going great iv'e gotten them all lost now on level 6 :)