WASHINGTON—More than six years into an Internal Revenue Service clampdown on what the agency says are abuses of land-conservation tax incentives, the deals keep coming.

IRS officials had hoped to shut down the most aggressive operations in a mini-industry built around tax breaks. Instead, they are in a protracted wrestling match with deal promoters. Government scrutiny pushed out some deal makers, but that ceded the market to others, who set aside reserves for legal expenses and risks and bet on beating the IRS through litigation.