You will not keep spotters, or more particularly dog walkers off the strip. Ther short mown grass is very attractive to them and the concept of an aircraft using the grass is alien to most.
I speak with some experience of this issue - even to the point of dog walkers parking on the strip after compromising the gates.....
30m either side is a lot of space. Nice if you can get it but much more than most strips need (or indeed have!). Whatever 'space' you have will need to be mown to a similar standard to the runway, otherwise the long grass will actually pull you off the strip it you get one wheel in it.
With an 80m wide area you have got a lot of grass to look after. Which will mean at least a 5 gang mower and tractor, but preferably something bigger.
My last strip was 30m in total and we had no issues with clearance with low wing aircraft - but only a fence on one side. Still took over 1.5hr to cut with a 3 gang mower!
The majority of strip flyers are rather better at staying on the centreline than many others - but for obvious reasons. Obstacles which are more solid such as trees, walls etc tend to be given a natural level of clearance. Think about how close you would be happy to get - usually about 1 wingspan is 'fat and happy'