Our gateways have been included into a community that is not ours because that community boundary / range has been set very wide.
How do we remove our gateway from their community or block them from picking it up?
You can’t Rob, and frankly it makes no significant difference to either your or their community - they dont get any access to the GW - only the owner or nominated collaborators do, everything else is in the public domain anyhow and everyones node traffic in range is passed through to the TTN back end and only decrypted/accessible by ‘your’ keys etc. 
In some cases communities extend due to outlier gw’s or users (that has happened over time with some I have set up and have seen with others) but as shape can only be set as a circle of radius x meters other areas get ‘captured’. You may also want to have super sets of communities e.g. a town community may fall inside or be part of a regional or county level community so Reading community may exist the a larger Berkshire community then a larger still Thames Valley or what ever (made up but you get the drift). Down your way (S.West UK) I can imagine say a Wadebridge local community being a ‘contributor’ to the wider area regional SWIN community also some sparcely populated areas around the world may have huge regions to get their numbers up and facilitate collaborations (
ChristChurch NZ was about 250km last year IIRC). London community is huge - but London esp as demarked by say M25 orbital motorway isnt a perfect circle and to include some extended areas many of my and others GW’s in BE&C community end up ‘captured’ and added to London GW count. Last year on vacation with a pop-up in Midlaren near Groningen in The Netherlands the GW was picked up by a regional (effectively large county) community dozens of km to the south that had to be that large to capture the outer regions of the ‘county’. Note also some communities deliberately extend to capture more to get official status faster 
(I’ve also heard of some personality/philosophical disagreements between members of different communities making this situation a little gauling but thats just the way it is).
If you have a specific issue or suspect some abuse let the leaders/management of TTN or one of the moderators of the forum know by PM otherwise I wouldnt sweat it
Only problem arises when a new Commmunity gets set up without realising/checking there is an existing one just with a different name (usually for historic reasons) that already covers their area and then GW and contributor may potentially get split delaying time to being ‘official’…
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