I would suggest being as 'non-confrontational' as possible.
The vast majority of people simply do not understand the risk of walking on a strip, so a polite notice along the lines of 'this is an active grass runway aircraft operations can hazard people and animals' is probably enough. fences of course do help.
For the 'hard core' then a suitable letter from your solicitor direct to the person(s) is unlikely to make things any worst and will serve to give you grounds / justification for further action.
But you are starting on the back foot, like it or not, getting on with the neighbours is fairly necessary, once sides have been drawn then it is very difficult to recover the situation.
I would suggest thinking carefully about what has provoked this situation and see if there is something which you should definitely avoid doing in the future or possible hope to recover from.