You can designate a parking spot in PF3 under the tab "Designated Parking Control" on Options#2 button, enter ICAO code ie: EGLL, hit return button and select from list of gates/parking, make sure you don't have "Gate Parking" selected/checked on Options 2 tab under "Advanced Options and Tweaks"...otherwise you will get default Taxi to Terminal.
You can also alter the afcad file with an editor suitable for your Windows O/S not AFX 1.10 and Windows 10 as it just doesn't play ball...but something similar,this would mean having to designate the particular gate with the type of aircraft and maybe alter the parameters, see the manual on designated parking...you wouldn't necessary stop AI from parking in your spot but it may help to stop it most of the time and give you the space you want.
Or another thing you could try... if you have traffic turned up high, then turn it low that way you may also have a chance of grabbing a parking spot.