The group said the move had been made to protect the interests of existing investors and to avoid potential performance dilution.
From 30 November no concessionary terms will be available to new investors and the full 5% initial investment charge will apply, but existing regular savers will not be affected by the changes.
Henderson said at inception the strategy set capacity at $2bn to ensure Wallace and Newman were able to invest only in opportunities that fitted the investment objective of the fund to provide a positive return in all market conditions.
The size of the Absolute Return Oeic is now £368m, while the Sicav is approximately £340m and the hedge fund within the same strategy stable is £380m.
Added to these funds are the managed accounts, which takes the combined total under the strategy close to capacity.
Simon Hillenbrand, head of UK retail, said: "Since the launch of the UK Absolute Return Oeic in 2009 and the Henderson Gartmore UK Absolute Return Sicav in 2010 the funds have demonstrated they are capable of delivering positive returns despite challenging market conditions. However, as the funds have grown in size, we are mindful that there are certain capacity constraints.
"In order to ensure the strategy remains nimble and to protect the interests of existing investors and future performance we have taken the decision to ‘soft close’."
The fund returned 1.2% in the year to 30 September and 2.2% in the year before that.