right well, it looks like your arena is pretty much square...
so you just need to set boundaries like this
IF X < 0 then X = 0
IF Z > 100 then Z = 100
x being your player's x position
this will make it so that if the x co-ordinate goes past this point
it will not allow it to go any further!
you can change 0 and 100 to any value, and do it to any variable (like your Z co-ordinate for the other 2 walls)
this is just VERY simple collision
you need more advanced commands for detecting it among shapes etc.
but a quick search of the forum will yield this
also, i do not reccomend DBPro's internal collision, i haven't used it in ages, but last time i did, it was horribly slow and buggy
try other people's collision commands, and when you have more of a grip of the language, try some math collision, which is just an expanded version of what i showed you! i use this in all my games, and it is lightning fast and if you program it into a function can be very versitile.