The pavilion in Shroggs Park, Lee Mount, now has metal sheeting and security screws on all windows and doors after a padlock was found cut off and the front door open.
Calderdale Council says it has taken the steps to secure and protect the pavilion.
Campaigners are trying to save the pavilion, which used to provide changing facilities and storage for local sports teams but is now in desperate need of repairs.
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They voiced concern back in April about the council securing the building and only allowing supervised access, worrying it would setback their bid to have it reopened.
But the council has insisted no decisions have yet been made about the building's future.
Councillor Silvia Dacre, Calderdale Council’s Cabinet Member for Resources, said: “It is extremely disappointing to see a valued community asset vandalised.
"The building is already in poor condition, and our priority is to keep people safe and to protect the building for possible future use.
“People visiting the park will see new security measures at the pavilion. We want to reassure them that this is for safety reasons in response to the recent break-in, and is not part of any wider plans for the building as no decisions have yet been made about its future.”
Councillor Jenny Lynn, the council’s Cabinet Member for Public Services and Communities, added: “We are committed to the future of Shroggs Park, as we know how much local people value the facilities for their health and wellbeing.
“We’re delighted to be working with community groups to plan a range of improvements throughout the park, including possible options for bringing the pavilion back into use, which will be led by the local people who know it best.”