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It will take a lot of effort in time and money for negative SEO to have an impact.  I would suggest contacting the site and request the link/s be removed if the site is not relevant and spammy.  At the same time to negate any negative linking you should be continually developing your link profile so any single instances like this are watered down by an excellent profile.

D surely is an option to take into account (see my answer just above). Said that, I would monitor constantly the % of traffic from BE and NL and if one of them starts being relevant, I would use that data so to propose the migration to a .be and/or .nl domain name. Remember that, in that case, you will also start thinking in implementing the rel="alternate" hreflang="x-X", in order to solve the duplicate content issue between the two sites.