Should it be the Pitstop Menu at the right when you leave the pitlane ?
In this case, I'm not sure you may disable it .... but you can shorten the time it remains open.
( I think it's 10 seconds by default ).
To change this visibility time, go to ....\Steamapps\steamapps\common\assettocorsa\system\cfg\pitstop.ini
Open it with notepad and at the top you have:
[SETTINGS]
USE_MOUSE_PITSTOP=0
VISIBILITY_MAX_TIME=5
ITEM_FONT_SCALE=1
STAY_IN_CAR=0
AUTO_APP_ON_PITLANE=1
Just mofify the line in bold here above at the wished value in seconds.... and save.
5 seconds is a good value if you need it on track for choosing or modifying pitstop presets while driving on track.
Shorter time ... might be short when driving on a track with short straights ... it should take too much time to change easily a preset ... and this menu should disappear too quickly.
NB: if you absolutely don't need it ... try this value at 0